Just tell her that you just wanna stay friends. And you can even say well we can be close and really good friends but I just don't think that I'm interested in being in a relationship with you. I hope this helps you!
Don't flirt, don't encourage her, don't give mixed signals & always see her in a group setting so there is always people around. Deffinately keep your lips apart from hers & dont sleep with her. She will eventually get the message without being humiliated. Simple.