How would you handle this situation? (Guy hitting on girlfriend)?

Tell him that she's taken and to leave her alone.

Include that she's not interested in him at all. (Did she ever tell him that? If not, have her try it first. If he's still persistent, go tell him whatever I said above.)

If he refuses to believe you, then bring your girlfriend with you and have her confirm that she's definitely not interested in him at all. (If he knows that she's completely uninterested, he won't bother trying to get her attention.)

Hopefully it doesn't go too extreme, but if it DOES escalate to a fight, defend yourself. (If he keeps on trying to punch you or something, then just fight back -- only enough to make him stop.) If he wants to fight you, there's no way he'll stop bothering your girlfriend unless you show him that you're serious. (Hopefully there's no fight though..)

I hope this somewhat helped you? :)

you could communicate to him without being confrontational approximately it. clarify to him which you and her are jointly and he desires to go away her on my own. If he persists or desires to start a combat then......