Help with the dating situation.?

I’ve been dating for a few months now, and one person has become a real pain. I’ll spare his name, but he has been desperately trying to get with my girlfriend. It’s probably important to tell that before I had asked my girlfriend out, we had mutual feelings about each other. This person was flirting with her before we were dating, and continued during our relationship. I never did anything about it. Recently a fiasco went down where apparently one of this persons friends told him I was dating my girlfriend, and he proceeded to be a total arsewipe, and punched walls throughout the school.
Keep in mind that my girlfriend knew he liked her but kept him in a friend zone, despite him making moves ALL THE TIME. After the fiasco, he reached out to her, after being an ****, and she blocked him on one form of social media. He then reached out again on another form of social media, and bullshitted his way back into the friendzone. I know that he will continue to flirt with my girlfriend, and the strange thing is, when my girlfriend told me that they were “friends again” she seemed pretty happy about it. I want to tell this person to piss off, but that would come off as possessive, and since they are in contact again, they would defiantly talk about it. I’m also pissed at my girlfriend for letting this happen. This person is untrustworthy, an attention whore, a liar, a ****, and has zero respect for me and my relationship. Someone just give me ideas please