I had this problem when I first bought my land. Tried being nice and asking, that did not work. Tried posting, that did not work. Dropped trees across entrances, that did not work. So I waited on the property until I caught them, explained that mext time I caught them there I was going to be unpleasant, that did not work. Waited and caught them again and met them with a baseball bat, told them in no uncertain terms next time I catch them they were not leaving without broken bones and smashed atv's. One guy got lippy so we settled it the way men do and I have not had a problem since. Guess the word got around.