That's really shocking, Sol, and I think you did the right thing. By walking out in the middle of the first set, rather than politely waiting until the intermission, those "protesters" were making their own political statement. If they had wanted to discuss the issues with the performers, they should have stayed and discussed it with them, rather than asking you to forward emails asking for their money.

Let them sue. Sounds like they may have even gone with that intention in the first place, in this election campaign climate. They can't possibly win IMHO, and it will bring much publicity to your house concert series.