First I'd make sure I had all my ducks in a row as far as postings,warning,notice etc. Whatever your state requires for trespassing Then I'd get pictures of them trespassing then I would go have a little sit down with them and ask them if we could handle it like adults or if I needed to take it further.

The bottom line is if you just go to the law you will probably fix the trespassing but the rest will likely get even worse. Handling it without the law would be options A and B for me.....C would be the law