RE: How to deal with tresspassers??
Your state has to have a law of Defiant Criminal Trespass and then another code from Defiant Criminal Trespass (which would cover ATV's). I would call the local police and obtain a copy of it as I have for Pennsylvania. Once you have it, I would highlight the code along with a letter explaining to this person he is not to be on that parcel. Get deliverly confirmation. In PA, notice to a person can be in writing, verbal, posting and even a gate is considered notice.
Having sent a letter with deliverly confirmation will prove your notification to him, along with the posting. In addition, keep a record of his trespass...times & dates.
Being polite never works, he is doing so since nothing has happened. Once he pays a fine and is charged, he will then decide to change his mind.
Personally I have no use for these type of people. Somehow they think they have the right to do so, which is totally wrong. How would he feel that just before you went hunting, you walk into his house, use his bathroom and grab a snack ??? No different in my opinion.
Good luck in dealing with this situation