One of my high school friends sent me a nasty message on facebook? please help?

[Ryan S]

Here is the message my friend Marni McMillin had written "Ryan keep this bullshit up with Mariza and you're a dead man, grow up and sort your shit out before someone does it for you." I really dont understand why Marni is saying this to me. I dont know what she is talking about but I do know who Mariza is. She went to our high school with me and we didn't really get along very well. By the end of high school Mariza and I had made absolutely no contact with each other. However both girls Marni and Mariza were pretty popular. But I am scared half to death now, I just logged in to my facebook and Marni wrote it on my wall. Any suggestions on what I should do? Do I respond to her message or do I delete Marni off my facebook and block her. I've already messaged my other friends and sent them the horrible message that Marni had sent. Some said to ignore it, Some said to go to the cops. I dont know what to do but do I have a right to not be threatened like this on facebook??? Or is Marni just starting high school drama for fun? We graduated in June 2009 so I have never done anything to Mariza or contacted her for almost 2 years. Any support and help will be appreciated.