My girlfriend and her ex are still friends. How do I stop wanting to punch him in his stupid face?

You need to put down boundaries, because this situation is not healthy. Tell your girlfriend that you won't hang out with her if her ex is there. If she asks you why, tell her the truth: that you can't stand the guy and want to punch him. It looks to me that your girlfriend likes to tease both her ex and you. But what is she getting out of it? Is it her own insecurity that she's trying to cover up? Maybe you need to ask her. She needs to make up her mind, once and for all. Don't be scared to give her an ultimatum. In the end, she will respect you more for it. Good luck.