I had a similar problem with a neighbor accross from me with 4 wheelers. I sternly warned them...and I am mean sternly with a shotgun in hand. However, that didn't solve the the problem. I caught them a 2nd time and told them if I caught them again I was going to call the sherriff. For piece of mind a put up a barb wire fence and posted no tresspassing signs....problem no solved. It depends on your state, but if the property is not marked no tresspassing, you don't have much of a leg to stand on. Good luck.