Talk it out. If the flirt is a friend or someone you know well, sit down and have a conversation with him, letting him know how you feel, suggests psychologist Irene S. Levine on the "Psychology Today" website. Explain that his behavior isn't appropriate or acceptable. It's possible that he just thinks his actions are friendly, and not flirty. Clearly tell him, "You've crossed the line, and you need to step back and stop flirting with my girlfriend."

Limit the time that you and your girl spend around him. For example, if the flirt works at the coffee house in town, choose a different hang-out spot. If he's a friend, start spending more "alone time" with your girlfriend and scale back on the group outings until he gets the message that his actions are affecting your relationship with him and with her.

Act assertively, not aggressively. This is of particular importance if you don't know the other guy. A friend may understand that you are simply standing up for yourself, while a stranger may interpret your actions in a different way or think that you're looking for a fight. Speak in a clear and direct fashion, while remaining calm. Give the other guy some space, and don't get right up in front of his face. Avoid harsh words or sarcasm, as those come off as aggressive. Let him know that you are her boyfriend and that he needs to leave her alone. For example, you're at a club with your girl when another guy starts chatting her up. Say something such as, "She's with me. I'd appreciate it if you would back off and leave her alone."