Telling her not to talk to him wasn't great because it signals distrust to her, unfortunately people hitting on your girlfriend is something that's going to happen all the time, and there's nothing that you can really do about it apart from demonstrate high value by not reacting negatively to it.
Just the other day my gf was telling me about how her housemate asked her the morning after the party what the chances and some guy who "really likes her" ever getting together were (he was in the room as were other people). And how the same housemate was talking to a different guy on facebook chat(who also really likes her and is her friend) about all the things he's like to "do" to my gf.
Now of course there's that caveman part of my brain that would like nothing more than to simply do these guys in, but that's nither helpful or the way forward. What matters is that my girlfriend is applalled by her housemates behaviour, and that she did nothing to encourage these people. My respose to her was that her housemate's behaviour wasn't particularly nice.
She told me not to worry about it, to which i replied that i wasn't worried, i trusted her and besides who can blame them because she is really hot after all. But they can't have her
My point here is that your gf acted perfectly in your situation as did mine, she discouraged his behaviour, and told you about it. Thus no jealousy is warrented, I think you should remove the "ban" on talking to this guy (who cares' he's an AFC loser anyway) because this will show her that you do trust her, and that she can talk to you about such things in the future
