Most Helpful Guy
Don't do that, Serena. As tempting as it may be, if he does in fact approach you, just simply say "hi" and walk away. If he tries talking with you, just be short and say you have to go and walk to the other side of the place.
Then, if he continues to pester you or be a dick, go ahead and tell him to f-off.
Most Helpful Girl
Go with others. If you can go with a guy. If he does approach you just smile and say hi and put all your attention into the people you are with. The best thing you can do is to not let him know that he's hurt you and still has some sort of affect on you. It is hard but it is doable. The thing with some exs is that they absolutely hate to see you doing well especially if they're not.