Analyze your relationship with your girlfriend. Look at how she handles flirting and realize whether or not you feel confident and comfortable with the way she handles it. If she flirts back, talk about it and let her know what behaviors are uncomfortable for you. If she simply ignores the flirting, let her know you appreciate that she does not flirt back.
Distinguish between flirting and other behaviors. Many guys and girls cannot tell the difference in whether someone is flirting or simply being friendly. If a guy is just being nice to your girlfriend, don't get upset. However, if he is flirting, then you may have a reason to feel uncomfortable with it.
Show affection to your girlfriend. If the guy flirting with your girlfriend does not realize that you are her boyfriend, he may assume she is available. If he realizes that she is with someone, he may control his actions more. Put your arm around her, kiss her and talk to her. If you are in a bar, ask her to dance. Talk closely to her, but do not smother her or make it too obvious that this is the only reason for your behavior.
Avoid getting angry. Try to stay cool when confronted with a guy making moves on your girl. Once he realizes she is with you, he should stop the flirting. If it continues, try to remain calm look for peaceful ways to handle the situation.
Say something to him. If this guy is persistent and continues to flirt, go up to him and politely tell him that she is your girlfriend and ask him to stop flirting with her. He probably will not expect you to say this to him, but it is better than starting a fight. Be nice when saying it to him and say something like “Hey, that is my girlfriend, would you mind not hitting on her?”
Leave the location and find a location where you can enjoy one another. If the guy continues to flirt anyway, ask your girlfriend if she'd rather go somewhere else.