What Should I Do About This Cyber Bully?

[sha]

There is this girl who I thought was my friend in 2010. We used to talk and everything and joke. One day out of the blue sky, I found out that she didn't like me. I found out she was talking crap about me to all of my enemies and claims she has some private information on me to spill to everybody on Twitter. I only seen her 1 time in real life and when I did, she had nothing to say. She was standing in line and she just looked at me with a really nasty attitude on her face and looked away after I caught her staring. I have argued back with her in the past, like last year. This year, all she has done, was sit on her Twitter and talk about me. She says that I'm not important and that she rarely thinks about me but that's all she does. She comes to my Twitter and comments on the things I tweet and do. A week ago, she told me that maybe one day we could catch up. Now, a few days ago, she's back to insulting me. I asked her again what her problem was and she said I was all things things that I'm not. She even said I had low self esteem, which is not the case. She has that. She accuses me of doing everything she does. I'm sick of her talking about me. I know I can protect my tweets and block her, but the fact I even have to protect my tweets sucks. I can't tweet certain people that aren't following me if I lock my tweets up. I am a strong person, but I don't like to argue or to insult people. I'd rather just get along. What should I do about this? I let her know that whatever private information she thinks she has of me, and I don't know what, I told her to say whatever she wants to say because I'm over her threatening me. She doesn't know my phone number or where I live.


Sometimes, I try to think about what celebrities have to go through to motive me
to just brush it off, but I'm not a celebrity, so that really doesn't help for long.
What should I do about this? I've tried to get along and everything.

...I cried writing this...
I really want some good advice..
I had my tweets protected for a few days and she still talked crap about me. What makes it even worse, is she's subtle about it. I can't stand subtle bullies because you can't contact police that way.
She doesn't put " @" when she tweets about me and to me, but in arguments she corresponds to what I say. She's gotten her friends to tweet at me and gang up on me in the past.