When I bought my land I was having problems with tresspassers. The gates had been destroyed, fences cut and lots of dumping.
I put in new gates, fixed the fences and posted with multiple signs. They kept coming, so I got the sherrif to increase there patrols. Next they dismantled my back gate to get in.
None of the neighbors now anything, or so they say, so my next move was to the local DEA agent who has some land down the street. I told him I was worried about somebody growing pot on my land, but I couldn't find it yet. He said I had a good place for it and it was possible. I then told the neighbors the DEA had access to my land to help me find the pot plants and they would be watching the place.
That was six months ago and I havn't had a tresspasser since. I think the word got around and scared off my problem. I hope so.