How do I get her to stop?

[Katie]

IF she is posting false accusations on face book or through other media, you can sue her for defimation of character. That is so much more gratifiying than beating her up and going to jail. Also, be aware that the first complaint is not always taken seriously but it is a start. Some people start crawling out of the woodwork (so to speak ) and will have their own dramas with her and may never speak up until they are waiting for THE BRAVE ONE to push through this.
Also, if you are of school age there are counselors there and they can help you and you will have that on record should it ever come to a physical situation between you both and this documentations by Authority figures will carry a lot of weight.

The pen is mightier than the sword.
anybody can hit anybody, and bruises and cuts and broken bones heal. But the right words just keep on in the head and that is the best revenge. and should put a stop to it if she is close to normal.