Depending on the state you live in you can get what is called an Intervention Order (Victoria) Apprehended Violence Order (NSW) or even a Stalking order.
I get the feeling that you still love her though, if you want to cut all ties to stop her, you need to talk to your family and apply for an order to stop her, The Magistrates court is where you need to go. You can do it yourself just go to the Family Violence Counter and tell them what your problem is.
If you live in Victoria look here and if not they are fairly similar. http://www.magistratescourt.vic.gov.au/CA256CD30010D864/page/Intervention+Orders-Intervention+Orders?OpenDocument&1=37-Intervention+Orders~&2=10-Intervention+Orders~&3=~#1
Try telling her the truth, that you do love her but...... behaviour etc .... at her age things probably won't change much though.
I would also give consideration to the fact if she knows you well enough, she may be reading you here, posibly even posting to you!
