I do not think you have to be a horse person to have the common sense that trespassing and feeding other people's animals without permission is wrong.
I do agree that educating is a great idea.
You would be amazed. "Common Sense" is unfortunately becoming a very uncommon commodity. Not only do they let tiny children swarm up my stone walls, in spite of the slipping and falling of the 80-lb. rocks, not to mention the signs--I once looked out to find a man and his dog disporting themselves on my front lawn with MY dogs; and he'd gotten out of his car to do it! Standing right by the corner of the porch like he owned the place!
For some reason "city" people (which is most of 'em!) seem to think the world is a Disney theme park for their kiddies and their dogs, and concepts like horses biting, let alone Private Property, don't even enter their consciousness. Ditto the ones who let their dogs bark at your horse out their car windows when you're riding on the roadside . . .
It seems that when it comes to animals, perfectly sensible people become soft brained jellyfish. For instance we were living once near a very large and wooded state park that developed a black bear problem. My neighbors who were both intelligent, sensible people thought that they were "so cute" that they were "leaving food out for the poor things". There isn't anything cute about a black bear out your kitchen window in the morning or 'begging' for treats in front of your car when you are trying to get home in the evening.
I don't know about your state but I'm pretty sure that in mine if I have hotwire in an area accessible to the public I am supposed to have a warning sign. I agree with the others that your best bet would be to talk to the people and put up signs but don't expect the signs to be 'noticed'. If it really becomes a problem and if it is feasible, you may need to put in a fast growing hedge or a secondary interior fence to make the horses less enticing.
My horses' pasture borders the road coming into our housing development and I once came up the road to see two women out there petting my mare. At least they weren't actually in the pasture, however, see one with a bag in her hand and I pull up to find out what's going on. She was trying to feed my horse seasoned croutons! Shoving them in her face like she's going to force her to eat them. I told them do not feed my horses strange food that might make them sick. She responded "oh it's okay, she didn't eat it." I said it's not okay, do not feed them at all. I think they thought I was a bitch, but I don't care, they're not the ones who would be stuck with a vet bill.
Yogurt - If you're so cultured, how come I never see you at the opera? Steven Colbert
Man I hate it when people do that. I had a horse once who simply could not figure out how to eat apples. He would mouth it, and eventually drop it to the ground, Imagine my surprise when one day he snatched a piece my friend offered him, ate it like he'd been doing it all his life and looked for more.
That made me suspicious. Sure enough, not three weeks later, while in the tack room one afternoon, I see a man and kid walk by the door to the back doors of the stalls carrying a bag. I had just put everyone in for the afternoon.
When I got outside, he was pulling an apple out and was giving one to each horse.
Long story short, my husband's clinic is on the site. This man had been coming in regularly for diabetic treatment/monitoring for his cat. He had taken to bringing apples and regularly feeding my horses...no permission, no asking or saying anything. Thank God, it was only apples, and that no one took his fingers off. I was polite with him, and he looked at me like I had three heads. And I have signs posted everyplace, signs of all kinds, but none of them made a difference. A real pain in the ass.
My horse grazes at a public park in the middle of suburbia. People are allowed to walk THROUGH the paddocks (with their dogs/children/whatever). There are signs at the external fences asking that they not feed horses. People scratched away the NOT so it read Please do feed the horses Generally I try to educate people. Most do get the idea when I ask them how they would like it if I walked into their house and gave their CHILDREN jaffas (chocolate lolly with red coloured candy shell). I also use the laminitis sort of like diabetes analogy and explain the need to control what food the horses are getting. It's not easy though.