How do i stop someone from self harming?

That requires: the help of psychiatric specialists which should be enlisted quickly before the situation gets out of hand. You can't do it alone.

Unless the: someone is you - you don't have control over that person. Dialectical Behavioral Therapy is the gold standard in this area. Take care.

Get her help.: You can help her stop by advising her to get professional help and maybe facilitate this from happening. She may not even know what to do about it because she's very young. There are crisis hot lines, the emergency room or even get help with her primary care physician who can refer her to the appropriate professionals. Her parents should be made aware also.