Please help! People's well being in stake! Stalker!!?

We've had a problem with a girl at my school for quite a while and now its a VERY big issue. We will call her Neidrin. Neidrin has a history of violence though for unknown reasons had not been expelled from my school. Ever since late November she "befriended" one of my closest friends whom we'll address as L whom shares a common interest in amime and video games with her and me. At first at wasn't a big deal but gradually she forced herself in our circle of friends and started sucking me and my other friend "G" into her lonely tidal pool since no one else was nice enough to reach out to her. We all really did try to accept her in our group but she has certain qualities that make her very uncomfortable to be around. For example she talks extremely fast making it impossible to understand, threatens and physically hurts others, laughs at others in pain, swears constantly, intimidates others, and never stops talking about personal problems and will interrupt a conversation to talk about the boy she forced to be in a relationship with and constantly harasses him with inappropriate swears and sexual terms. We've tried multiple times to tell her to please leave us alone or change her ghastly behavior. Whenever this happens she calls us and assortment of insulting names then walks away to complain to other victims who try to avoid her as much as possible. The next day she trudges over to us like nothing happened. We've talked to teachers and counselors about her behavior since its obvious she requires some help but no solutions or changes were made so it's time to take matters in our own hands. We've even tried to be completely bitter and cold aka jerks, weirdos, and exclude her from the conversation but she always clings to us and refuses to let go. I know this may sound cruel but its not are responsibility to help someone in desperate need for professional guidance. Does anyone have helpful ideas on how to solve this problem.