If I ever get the threat of being DF'd I think I will say :
" The Religion I left a few years ago no longer exists, you can't Disfellowship me from a religion that I never had fellowship in !".
I know this will make no difference at all, as would whatever I said if they had made up their minds, but it would point out how very very silly the whole idea would be.
Any other ideas to briefly sum up what a stupid thing DFing is ?
Not long rambling or scriptural, just a short, sharp barb or two that we could get in before slamming the door.