How to tell my girlfriend to leave this guy alone?

[James]

My girlfriend has this guy that is sort of her friend. He tries to talk to her sometimes and it bothers me because he used to like her. She isn't attracted to him but she always feels sorry for him and gets mad at me for talking bad about him. I try to show her that he bothers me but she thinks I am just being mean. Now this guy takes a lot of selfies and tweets a ton of girls. The day after we started datig there was a hashtag called #mentionsomeonebeautiful. So I mentioned my girlfriend. I looked at her previous tweets and this guy had used that hashtag on my girlfriend. So I deleted my tweet and got upset. Should I be upset at her? How can I get her to get rid of this guy before I physically do it?