So now, the serious part of yesterday's ideas.
People hate confrontation. The word itself sounds antagonistic, like there's going to be a knock-down, drag-out fight before you even start. In the SCA, this leads to nice people giving in to the jerks who always yell and whine until they get their way, no matter how many nice people get driven out by their behaviour.
It's not an SCA issue, it's a societal issue. The US has cultivated a couple of generations of people who genuinely believe that they are entitled to anything they want, and that to be told "no" damages their self-esteem, and is therefore bad. I've seen this entitlement attitude described on forums, and I've seen people pull it - especially the "it's rude to tell me I'm being rude" attitude - on etiquette forums, of all places.
In an SCA context, this results in the severe abuse of the goodwill and generosity of some people, who are actually afraid to tell an asshole that they're being rude. For all our vaunted "courtesy", there is quite a subset of people who display very little courtesy, but are super-quick to accuse other people of the lack of it if they are asked not to do something. They wield the word "courtesy" like a bludgeon, laying waste to anyone who thwarts their desires, and making a huge fuss until everyone give in, either to get some peace, or because they're afraid of being slandered.
In the field of hospitality, we call these jerks moochers.
First, let us define what a moocher is, and is not: They are not people who occasionally slip up and forget to vacate a chair, or ask for tent space, or get stuck, and have to ask for food/water/a ride. If you give back as much as you take, you are not a moocher, but part of the generous spirit of the SCA. Pay it forward or pay it back, you pull your weight most of the time.
No, moochers are the people who have a pattern of taking, without ever contributing. They don't apologize for their behaviour, and get very nasty if called on it, turning the accusation around on the nice person who is trying to stop the abuse of their generosity. They expect to be catered to, they have excuses for everything, and justify their bad behaviour without ever learning from it. Everyone groans quietly when they come by, and everyone feels relief when they move on to the next victim. They never say please or thank you, and they are never grateful, since they consider whatever they want not only free for their use, but rightfully theirs for the taking.
This sort of thing is terribly difficult for most of us to deal with - we've been taught to be polite. More, we've been taught that there is a "social contract" - a code of behaviour that everyone follows, so that society can get along with minimal stress. When a mooch comes along, they break that social contract, and it's a shock. Understandably, many people feel totally paralyzed by these situations, because we are not generally trained to deal with such flagrant breaches of etiquette. We expect to be able to subtly hint, and have people pick up on that hint. When we get stuck with a mooch who steadfastly refuses to take the hint, and reacts in a hostile manner to anything more overt, we panic and don't know how to deal with it.
Moochers don't just break the social contract, they've learned to use it to their advantage, as the myth that "telling someone they're being rude is rude" illustrates. For example: Say you're at an event, you've gotten up to get something, and when you come back, someone is sitting in your chair. You ask for your chair back - and the person doesn't get up. Either you don't say anything and feel upset, or you try again, and the mooch gets hostile, trying to make you feel bad with such statements as "but I'm talking/need to rest/have back problems!". Circle around to option one again; don't say anything, and feel upset, or try option three, asking again, and risk being told, directly or obliquely, that you're just so freaking rude for wanting to sit in your own chair. This is upsetting, and the mooch knows it - a polite person would say "of course! I'm sorry!" and get up immediately. The mooch's hostile reaction is designed to make you give up and go away so that the mooch can continue to steal your chair.
We cannot survive as a generous hospitable society if we allow these people to continue to behave in this manner. People will stop bringing nice things to events, because every time they do, they get mooched on by people they don't even like (there's a reason why everyone describes these people as "slight acquaintances" or "strangers"; your friends don't treat you that way). Why bring nice food and nice chairs if some asshole is going to eat all of the food, throw their stuff all over your pavilion, and take over your chairs so that you can't even sit in your own chair when you are tired without risking being called names?
It is one of the great pleasures of eventing to provide a nice lunch, a shady spot, or even just cool water, and gather your friends together to enjoy it. It feels good to provide hospitality, and you can make great new friends with it. However, if the assholes get there first, you not only don't have food left for your friends, the assholes will drive them all away. Hospitality then becomes an exercise in endurance, with stress if you don't say anything, and stress if you do. Why would you want to let yourself in for that? Better to bring only enough for yourself, and refuse to provide hospitality.
It is not wrong to POLITELY correct someone who is being rude. It is also not the end of the world to have someone get mad at you for doing the right thing - they're a jerk; what do you care what they think? It's not like any of your friends will believe them, even if they do go around saying how mean you are; people judge actions as well as words, and if you are known for your hospitality, very few people will take them at their word. So what if they make a scene? It's not you making the scene, and they look like the jerk if they're yelling or being rude and you remain calm and polite. Staying polite is vitally important - you must never lower yourself to their level. Remain calm, and you will come out on top, since it's very hard to argue with someone who won't argue back.
It is needful that we start correcting this behaviour en masse - sending the message that the SCA is an "everyone contributes" enterprise helps new people to learn quickly about good event experiences, and allows people to bring their nice things without fear of having them be abused.
You can practice this sort of thing with your friends, if it makes you more comfortable - it's hard to stand up and say "no, this is not acceptable" without rehearsing it, especially if you're used to just shutting up and taking the abuse. We all want to be liked, and moochers know this - telling you you're mean for refusing to be taken advantage of is their way of silencing your protest. We need to let it be known far and wide that we will not be manipulated in this manner any longer. Hospitality is a two-way street.
I am here to tell you that it is okay to say "I'd like to sit in my chair, please". It is only right and proper to have control of your own space, and if someone is mucking it up for everyone else, then they need to be stopped. The needs of the many outweigh the convenience of the one. Everyone agrees to the social contract you put down, and anyone who doesn't is free to bring their own dayshade and food, where they may do whatever they please.
If you bring nice things, you will get moochers - whatever you have is nicer than anything they have, and the moocher mentality says "they should share their nice things with me, because I want them to". The fact that they are bringing nothing to the deal doesn't occur to them - all they see is nice stuff, and they want in on it. Telling them they cannot usually results in pouting and whining, but then, telling a two-year old they can't have candy before dinner gets the same pouting and whining. It is bad for everyone that the toddler gets the candy; consider the mooch in the same light, and just say no.
There are coping strategies you can use for moochers - check and see if there is an open pavilion/dayshade that is for general populace use, and direct them to that, or an open space around the list field. Check and see if there's a lunch available for the populace, or a lunch they can buy, or even if there are a couple of fast food places near the event. Yes, they should be finding this stuff out for themselves, but they're relying on snacking off your nice food table, so clearly they will not settle for something less. Having alternatives for them gives you a way to be helpful even as you're steering them away from your tent.
At such time as they have learned to ask politely, contribute services, or bring their own supplies, you can feel safe inviting them back (if you decide they're okay - you have no obligation to be their best friend).
The same goes for bumming rides - if you make it clear up front that they will be chipping in for gas (and, if possible, tell them how much; you can work out a rough estimate using mileage and tank capacity), then they will be on notice that while you are happy to help them out, they must reciprocate in some way. The social contract says that people will automatically seek to reciprocate, but moochers need to be told directly and clearly. Give them a loophole by being vague or subtle, and they will take it - after all, they've been used to getting things for free, no strings attached (if you don't count general dislike and the complete lack of real friends), and they will be completely deaf to anything short of telling them "this is how much the trip will cost; your share is [%] of that cost".
For most people you encounter, the standard social rules still apply - they will reciprocate, and you don't need to lay down rules. However, when someone breaks that contract, you are allowed to correct them. If you don't, they will not learn, and everyone suffers.
We need to all take a firm stand on this - if someone leaves their children with you, take them back. If you can't find the parents, take the kids to the autocrat. If someone is in your chair, ask for it back with a smile. If they won't budge, take a firm tone and tell them you would like to sit. Even moochers don't like to look like jerks. If someone starts taking your food/chair/mugs/whatever, ask them to stop, again, with a smile. You can state it nicely, but it is very important that you don't give in if they resist. Suggest the alternatives you have already scoped out. If someone leaves trash all over your pavilion, ask them to pick it up (more extreme cases can be dealt with by collecting their trash in a small bag and returning it to them). Ask for help loading/unloading, even if you have to chase after the moocher (they have a trick of disappearing as soon as there's work to be done). Don't give them leftovers for free.
All of this can be done with a smile, and politely. There is almost never a reason to yell, and if you maintain your cool, they will look like a jerk for getting angry at you.
Let me repeat over and over again: It is not rude to ask for your chair back. It is not rude to ask a stranger to stop eating your food. It is not rude to ask people to clean up after themselves. It is not rude to ask people to help. It is not rude to ask people to move. It is not rude to ask for gas money/food money/a contribution to the camp. It is not rude.
It is not rude to ask for basic courtesy from the people enjoying your hospitality.
Now - go forth armed with this knowledge, and resist the moochers. You'll enjoy events more, you'll have more time for real hospitality, and you'll be able to bring things to events in the secure knowledge that they will not be appropriated by jerks.