If you're the person that she's interested in, simply tell her that you don't feel that you are compatible with her. Be polite if you do run into her, but keep the conversation brief and don't discuss you personal life. If this girl is flirting and hitting on someone you're currently dating, there's little you can actually do. Any attempt you make to tell her to back off will only egg on her behavior, as she will feel she is getting to you and the relationship. The person she is hitting on has to be the one to tell her he's not interested or to ignore her entirely. If they refuse to do this or worse, seem to welcome the attention, it's time to reconsider the relationship.
Raiyaz's Response, Woah, I think I see a problem. You need to tell her (politely) that she needs to back off. Depending on the obsession and how 'far-off' your boyfriend is, you might have to tell him too.
Ask them politely to give you some personal space and if they don't then just tell them to back off and avoid them.
He's probably gay, or he's trying to politely tell you to back off.
I suggest starting off with listing off all the reasons why your friends or why you like her, and tell her shes pretty. If she laughs at you or is rude then just brush it off your shoulders back off and find a new girl that is into you.
Tell the girl how you feel about her so you can get it off your back.
ignore her or tell her to back off
Just tell her to back off your girlfriend. If she understands that well she will back off herself. Otherwise you have to threaten her more.
You need to tell your friend that you also like this girl. If he understands, then maybe he will back off. If he doesn't back off, then you two need to talk. Maybe neither of you get the girl? you don't want to ruin a friendship over a girl.
Politely ask your cousin what she is doing, and then if that doesn't work, tell her that he is your boyfriend and not hers so she needs to back off and stay away from him.
SLAP HER! and tell her to back off. x] Tell her to back off and if she doesnt tell her shes not ure friend anymore. the best way : )
Eye contact is the key. Look them straight in the eye, without being manacing, then tell them firmly (but without raising your voice) that whatever it is they want to know is none of their business, and to BACK OFF!
Tell her how you feel about her. Tell her that you miss her and want her back. Do cute little things for her and tell her that whatever made you lose her in the first place will be different this time around. If she doesn't want to get back together, then don't get too stalkish, back off for a while. If you still want to get back with her then after you back off for a while talk to her as a friend. Maybe sometime more will happen, but if not try to find another girl that you like to try to keep your mind off this one.