What to do if someone you were once close to spreads rumors, shares secrets

It doesn’t matter if this was a friend or someone you were linked with romantically, if they start spilling your secrets once you break up, or are no longer friends, what do you do? first of all, don’t act defensive or upset in any way if someone tells you this person is talking behind your back. Basically blow it off, and say, “Oh, you’re going to believe that idiot?” And then change the subject. Neither confirm nor deny. Just act like nothing he/she says could possibly bother you.