How Do I Get Along With My Girlfriend’s Boyfriend that I Can’t Stand?

My girlfriend has this boyfriend that she's been dating on and off that I can't stand! The guy broke her heart and she ended up at my house crying her eyes out! After some time apart she has started seeing this guy again and it really worries me. I think this guys a player and hasn't changed one single bit!

I see this guy when I'm out and about from time to time. He's usually with a group of other people that know me, so I'll say "hi" to them and purposely give this guy the cold shoulder. It obviously bothers him because he makes comments to my girlfriend about how I don't like him. I know this is some what childish, but when you wrong someone close to me I'm not going to let you forget it.

How do I deal with this guy and not push my friend away?