That sucks, I hate to hear it. There is not really anything you can do, but if you do catch trespassers I would definately prosecute them. I know in Arkansas where I live and hunt, if someone is caught tresspassing/hunting on someone's lease and are caught they have to pay each member of the lease a certain amount of money, I can't remember the exact fee, but I'm sure if that is the case anywhere else that would discourage repeat offending!