Well it seems that she doesn't feel anything for you romantically and two rejections are indicative of her choices. The fact that she said "Yes" once makes me believe she has some glimmer of hope for you. So here is what I would do. I would totally ditch her and cease communication with her. If she honestly cares about you, she'll want to know why you've stopped communicating with her. If not, she has little-no interest in you and you should just ignore her.
I know the pain of rejection, and I know that once a girl doesn't love you, it is almost futile trying. If you are good friends with her, keep her, if not, have nothing to do with her after she shows her true colors, like I said with the example above.