The police are your friends sometimes. Has she ever threatened violence against you or herself?
Restraining order FTW.
Second choice is to change your number. Or have a chick friend record you a new voicemail greeting, like, "Hi! You've reached Chron and Susie! We can't come to the phone! LEave a Message!! (perky voice a plus)"
Or tell her you are gay. And that you were only dating her because your mom was hassling you to get a girlfriend. Be nice about it. See, if she is as insecure as she sounds, that lets her off the hook- if she needs validation from men, you dumping her because you don't like her SUCKS for her. But you dumping her because you're gay, she can get her twisted little mind around that and get enough closure to get the FUCK off your back.
Creepy!