Steve,
Have you been to the RV camp site? If that is where you think the trouble is starting from (probably right on that thought), then it would be a good place to go to try and stop them from going in the first place. I would also be on the horn to Keweenaw County Sheriff and the DNR every day complaining about it, tell the sheriff to stop by the campground and warn those to stay off private land and to make their presence known out that way on the weekends or when most of the problems happen.
I am on the board of directors for Keweenaw Trail Services (ones who oversee the grooming and maintainience of the trails) and I am sure we would be happy to help you out with some fencing, but just a heads up. All that will do is cause them to go around the fences and bring about more damage to your land. I have seen ATV's travel a 100 yards or more through the bush to get around gates and fences. It's just a few that do it, but before long you have a path that others think is the proper way to go.
Just my opinion, but closing down the snowmobile trail is a misdirected act. You will just be punishing law abiding snowmobilers and the ATV'ers that are breaking the law will continue to do so for years and years, whether there is a snowmobile trail that goes through there in the winter or not.
Send me an e mail and I would be more than happy to take the lead in getting KTS to work with you.
-John