My ex best friend won't leave me alone?

Me and this girl put an end to our friendship in July, I think.

I'm honest, I didn't care about her anymore and found new people I really like, I have my reasons because she hurt me a lot in the past many times and would do the same without even noticing. I've been disliking her since then. I told her to move on and explained I wanted to be away from her, which is the best for both us. However, she didn't accept it.

Since then, she has been trying to talk to me, by saying really rude things, criticizing my options, giving me warnings and even saying bullshit to our friends, like "I'm going to "ruin" her life". I already told her thousands of times that she should leave me alone and I didn't want to talk, but she's not doing that.
At school, she doesn't talk to me, which is good, but she's planning a "conspiration" with an ex friend of mine, by talking shit about me.
There was a time she wanted "peace" and no arguments and I accepted it, but she confessed to a friend that I still deserved a big slap. Also, she made me believe that our friends in common were against me but it was a lie.

I don't want anything to do with her anymore, but she seems really obsess. I'm also afraid she might influence other friends because of that "conspiration".
What should I do to make her stop?
My ex best friend won't leave me alone?
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