How do you deal with your ex saying bad things about you to the person your interested in?
Tell the person that you have a crush on that obviously your ex
is not over you, so that's why he was trying to ruin your future
relationships. Then, approach your ex and tell him that you would
appreciate him not trying to win you back. He'd probably get all
weird and deny that he was trying it, but in a sarcastic tone say,
"Ohhh, okay, whatever you say, <ex's name>." Then walk away
but DON'T TALK TO FRIENDS! Talk to your crush instead, making sure
to look over at your ex; if possible; and laugh, even if the joke
wasn't about him. This will show him that two can play at that
game.
ALTERNATIVE SOLUTION: Ignore it. When or if your crush asks you
about it, tell him the truth. If he brushes it off, just avoid the
topic and talk about your crush's fav things. If he doesn't brush
it off, then he can't accept you for who you are and maybe he's not
the best guy for you. =) hope this helps!