I don't want to help this girl anymore, what should I do?

Okay, so I'm writing a fanfic (fictional story invovving a certain band) With some girl that was online and someone gave them my sn that reads mine on a forum. Anyway, I need to get out of it cause I have no time for my own stories anymore and it is going nowhere, and she expects me to write everytime we are both online? I will not black her cause I won't sink that low, seeing as she did nothing wrong. What should I do or say, please help!