Get the frequency sharing plan in place. Get the property owner to sign it. Buy a frequency monitor. If one of them violates the frequency sharing agreement and shoots down a plane, take the property owner to small claims. Make sure you get witnesses, and prove that they are operating on the wrong frequency in violation of the agreement.

You may not prevail, but the property owner may be ticked off enough to either enforce the agreement or kick them off. If the club has been there for 15 years, theres probably something working in your favor.

Good Luck.
Brad