I feel I need to stop my girlfriend from hanging out with someone.
Why don't you want your girlfriend to hang out with this person?
I think this person is a bad influence on her.
It's not appropriate to be so controlling of your girlfriend. She's allowed to hang out with whomever she wants, and you can't tell her not to. If you keep trying to tell her what to do then she'll probably get annoyed with you and want to break up.
Fuck you! What are you trying to tell me?
I am trying to explain you can't tell adults who to see or what to do. She can have her own friends, you come off as controlling and that is really unhealthy.