Expose what they want hidden.
Beat them to the punch. Put yours in a good light and theirs in a bad one. Otherwise, you are already compromised and will have to hit the ignore button.
Make them believe that their most precious thing whatever that might be has been destroyed...or key their car
Male or female friends?
Anonymously expose their secrets
just make some real friends
First, I'd have to ask what they did. I have many "so-called friends" who never did me wrong, so they don't need any form of revenge.