Most Helpful Guy
If you don't want to talk to him don't start any conversation. If he does then just be polite and excuse yourself the first chance you can. I understand the feeling but the key here is just to move on, and bring up as few bad memories as possible. Why exactly did you two break up anyway?
Most Helpful Girl
Completely ignoring is gonna be weird as hell, and might backfire. Believe me. I tried it once and it went very bad. He wanted to say hi, his friends were there, I ignored him, although I forced it, his friends laughed, and then he started to tell them all kinds of sex stuff, like how took I it doggystyle and how I moaned like a slut when sucking his dick, all kinds of degrading stuff to make him look cool again in front of his friends and make me look like a whore. That was weak of him, I don't know if your ex would be that weak, but just be careful with hurting someone. They might hurt back a lot harder.