No one wins in a situation like this .

I would think that you need to think this out a bit more.

He has a membership card ,so what, it's a piece of paper.
Do you really need it back ?

I would think that the bigger question is ,would this individual
drive by with his transmitter(s) on just to be vendictive even if
he gets his dues back ?

IMHO if giving his money back makes the whole problem go
away in a clean fashion.......do the refund, it will be worth it
in the long run.

Good luck,
Roby