When a relationship ends, one party may still harbor romantic sentiments for the former and attempt to maintain touch with her. This can cause challenges in new relationships as well as resentment or hostility. If your girlfriend's ex-boyfriend refuses to leave her alone, attempt to persuade him that his behaviors are unappreciated and unwelcome. Remind him that she is now available for new friendships and opportunities. If he continues to pursue the relationship, then show him the door. The end of a relationship should not be used as a weapon against another person. If your girlfriend's ex-boyfriend causes you stress or trouble, then you should talk with him directly. Explain that his actions are hurting you and your feelings, and request that he change his behavior. If he does not, then you should consider whether the relationship is worth the hassle it is causing you.