Fritzy, your solution is the best.
But I disagree that the rogues aren't liable. If they do monetary damage they can be sued if you have one of those $50.00 scanners and can prove they were operating on your channel.
One question might be one of whether or not the other group are even AMA members or is this just a conjecture?
AMA has no business being a police organization and no one wants it that way and as far as I can see, there is nothing they can do for any of us in a situation like this.
So get a mouthpiece to draw up a letter to the property owner who may indeed not want to get caught up in a mess like that and kick them off.
We recently went through a problem where a splinter group broke with our club because they did not want to abide by AMA rules yet they got an AMA Charter and formed a new club.
A mower our club owned was driven off a cliff and totalled because they did not want to pay us for it. We refused to get into a battle with them. One can only wonder what goes through the minds of some folks. Just remember this: Never slap a man who chews tobacco.

3dbob