As others have said, call each and every time they step foot on your property. It may seem like a bitch move, but you have already talked to the guy so the next step is some sort of escalation. I think it is wise for that escalation to be through the law and not on your own.

And gather evidence!!! Film the dogs when they're on your property and if you can, film any dog fights. You may need it to cover your ass if you're forced to put one of them down.