What did repo lots and impound lots used to use, guard dogs? Any way to use wild animals or dogs effectively?
I have a little problem in the back woods behind our property that people joy ride, walk their dogs, snowski, use ATVs, etc on the 2 track back there because it was 'free reign' for years before we moved in and built. Signs don't help. I'm considering posting a sign on the most commonly traveled paths in and out stating 'tire hazard, sharp objects ahead' or 'enter at your own risk, spike strips in use' Then at least if they come in on anything motorized they will have tire damage and an expensive tow truck bill.
Probably any warning sign will be ignored. Booby traps, hidden banks of poison tipped arrow launchers will just produce retailiation.
Probably cameras and frequent patrols by law enforcement is about all you can do.