How Do I Get Over An Ex-Girlfriend?

I cannot just stop talking to her, i'm good friends with her brother. She lies and she is a manipulator. She sleeps around with other guys as well, but i still find it hard to walk away, I spend the night at her house with a bunch of friends every friday ( To hang out with her brother ) and still every night she gets me to sleep in her room with her and we end up messing around. i guess i just really don't know what to do, any ideas about how to get her to leave me alone?