I'm up north, and we have too many rules, but even when i lived down south, there was a minimum of 10-15 feet setback for anything on the property lines. Shortest I've heard is 5 feet.
I have my property posted, before we built anything had a lot of out of staters coming and hunting it and the adjoining owners didn't want them.
I asked my insurance agent and they actually said not to post it, that way you can claim ignorance if anything happens. What I've realized is, if you live in a rural area, people are going to wonder on your property. As long as they don't leave a mess/disturbance in their wake, I'm happy to let them pass through.
Every once and a while you get an idiot from out of town who drives past my house, to my mother in laws, keeps going and tries to set up a picnic on the creek behind the house. That's the type of person I am trying to keep off my land. But they are also the type of person who ignores posted signs. I've stopped posting things, and as they fall down, they won't be put back up.
edit: a neighbor of mine actually was set to take land from the previous owner through adverse possession, and ended up being a surprise to me when I bought the land. Luckily they traded like for like, but the risk is there, and its not entirely ignorant to post your land. Some people like OP will take advantage if you dont.