RE: Resolving close fields interference issues
I would appeal to their interest in keeping their own planes in one piece.
Sailplanes tend to fly a bit further away from their transmitters than power pilots do (I've been over a KM away, with my glider but can't imagine being that far with a power plane), and if the winds are up for tight in slope flying, the power field is likely not operating. I do think the gliders are taking the bigger risk here.
That said, I would urge them to move towards a shared fewquency system that would see clear separation of chanels and notifications of 'abnormal' events that may involve shutting down the other field (fly-ins etc)
If cooperation is not forthcoming, a mention of wreckless endangerment might be worth while. Now that they are aware of the potential for radio interference, by or to them, to continue without making some arangement would be inviting disaster and they could bear some criminal responsibility. UNfortunately, this could apply to both clubs.
Clearly, by setting up a flying site so close to an existing club field, they are voiding their MAAC insurance - if they are MAAC members.