If a performance shows no merit or gives you no enjoyment, if it was in your opinion unworthy of putting before the audience, don't clap

If enough people agree with you the perpetrator will slink off to the sound of their own footsteps. It is actually true that in a folk club people WILL clap anything

As not clapping at all is in itself bad folk club manners there are a number of devices that can be used without necessarily having to go to the loo - make sure you have a drink in your hand when they finish and gently pat the table or your knee, drop something on the floor so you have to pick it up, or if you clap using the boney parts of your palms it makes precious little noise. You retain your standing with other club mambers and have made your point