I agree with Fritz on this one. Everyone has a right to fly whether at an AMA field or on private property. I don't think the law recognizes the fact that you were established first.

I believe that you should invite the other gang to a meeting or at least have representatives from the two fields meet and work out a frequency sharing plan. They don't want their planes shot down either. It would be a win win agreement. There are plenty of frequencies to go around. You might want to take a poll of the frequencies your club members use . This way you can enter negotiations better prepared and knowing which frequencies are best for your club. You might find that the frequency conflict is very limited and easily resolved.

Make an effort to be kind and respectful. After all, we are all brothers sharing this great hobby. A professional demeanor and level headed approach will leave a lasting impression and might encourage the group to join your club some day. Any animosity at this point might perpetuate THE belief about AMA fields and the attitudes that prevail among them. This could result in affirming the groups' reasons for flying where they do. You catch more flies with honey.