Block her emails and any other form of communication. Keep all things she sends to you, and get a restraining order if you are in fear. The restraining order will allow the police to arrest her should she come around you.
My great neice is ranting/raving accusing me of things I did not do. She sent several ranting emails, one 3 pages long saying bad things about me. I refuse to join her in her rants. What can I do. I am getting worried about my safety?
You do t say how old she is. If she is a minor, call a meeting with her and her parents to sort this out. Take copies of rants and a timeline of her contact with you so her parents can see how outrageous she has been. You may learn from them that the girl is mentally ill and off her mess.
If she is an adult, tell her parents privately about these events, and that you could use advice from them about what to do next. Perhaps the three of you can agree on legal action, a restraining order, or something else. Perhaps they will move her away.