One major deterent for trespassers is to hang a couple of five gallon buckets or coffee cans in some trees where they are visible, and put a mess of bullet holes through them.
Keeping scumbags off your land is an ongoing effort. You can never rest on this. Once you run off a person or group of person, a new group always comes along.
It sounds like you need to go over the rules with these homeowners, in person. I would also have them call you if they ever see anyone on the land.
Excerpt from Robert Crais' "The Monkey's Raincoat:" "She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."
