Around here you might as well wipe yer butt with your posted signs. Only way you can keep people off is get really , really nasty when you catch em. If that doesn't work you have to press charges. It stinks and no one wants to be a Pr!^&, but people don't leave us much choice.
Had a bunch of kids tore up an old hay field a few years back. One was stuck in a mud hole. Told him my 1955 was about 600 yards away and if he wasn't gone by the time I got back with it, his pickup was going to become a pancake and a permanent part of the mudhole. Don't know how he got the thing out, but he was gone by the time I got back with the 1955. (smile)