how to fully stop my niece going on Facebook?

[Alex]

Talk to her. Ask her why she thinks it's okay to cyber attack someone like that. Perhaps she's dealing with bullying issues of her own. Maybe the people she's sending the messages to are hurting her, or maybe she was bullied without her family's knowledge in her past and now this is the only way she knows how to interact.

Don't try to just cut her off and assume it will do anything. The more you try to prevent something, the more you will see she will rebel to the power which takes away.

Even if she is 12, she can still be reasoned with. She still has empathy and emotion. Talk to her, if you can, show her how her messages affect the people she sends them to. Don't make her apologize until you have her side of the story, or you run the risk of forcing her to apologize to her tormentors. She'll never get over that.

Above all, respect her feelings of anger, but teach her better ways to deal with it.