Start an argument with someone that know. Insult them and make remarks at them until they let whatever you supposedly did. Sorry OP it sounds like they have you a favor... Forget them and find better friends. Real friends wouldn't keep it from you. They must be in the categories of: 1) mutual friends or 2) his friends. If any are 3) your friends, kick 'em to the curb. You'll find better OP.

Start some nasty rumors about your ex.

Well look on the bright side: you now have plausible deniability =P

start a rumor about how you broke up with her bc you caught her doing something really nasty. like your dog

Let them think what they want... You don't need any of them in your life if they believe anything anyone tells them!

Sounds to me like a prank, I'm sure at least SOMEONE would've told you what he said.

u know what u did... make something up :-o

I know this feels OP. You should try your best to stop it now before it gets too big. When I was in secondary school (for anyone outside of the UK, secondary school is between 12 and 18 years old) I went out with a girl who spread rumours about me when we broke up. The rumours had no basis at all but I was nearly excluded from the school by the senior management.

I also know how this feels. My sons dad did the same thing after I stopped caring that he cheated on me and left us for her ugly face. He spread rumors saying I wasn't letting him see our son then came to my house and said he used to have a son and doesn't wanna grow up, then spread more rumors and did the same thing your ex did and this was all at WORK. Burn bridges with those that are questioning your honesty and don't let it get to you and hopefully you'll find someone better and better friends. The best revenge is success.

They clearly aren't real friends