That's the simplest and best response.

I had (still have) a TC Helicon unit that had a preset (can't remember the name of the preset) that completely destroyed your vocals; made everything completely out of tune. We would turn it on whenever an unwanted 'guest vocalist' popped up on stage. The embarrassment and humiliation was usually enough discourage future 'guest appearances' but on at least several occasions, nearly started a fight when the 'guest' figured out what was going on. We did it once (as a joke) to our own vocalist in the middle of his favorite ballad. He didn't think it was funny and quit the band immediately after the gig. Oh well.

chas