Guys how would you handle this?

Just lookin for advice from a guys perspective..girls can chime in too i guess. My g/f has this new guy friend from her work, who is really sticking his nose into our relationship because he likes my g/f. Of course my g/f is complete niave and thinks it's just a friend being a friend. I don't know what he's doing just that he's laying it on thick when i'm not around. She gave him her number for something work related and now he is calling her all the time. Trying to get her to go to parties with him, to go out drinking with him, hang out, etc. I've been patient, i've tried to be a man about it and brush it off, but i'm getting annoyed now and am sick of him coming between us. He's disrespecting me big time. What's the best way to handle someone like this without making yourself look like a or jealous freak?