Is this person threatening to allege that you sexually assaulted her? And she is a person in another state whom you have never met?
How DO you know her?
What kind of proof could you possibly have that shows you do not know this person? I know I cannot possibly prove I do NOT know the President of France, so how can you PROVE you do not know the person making this claim?
Is she trying to extort something of value from you? If so, call the police. If she is just focused on you because she is delusional and saying she should call the cops on you, maybe you can call your local police and report her for your state's equivalent of harassing phone calls, e-mails, or texts (or however she is communicating this to you).