Annoying people problem. What to do?

A trivial yet very bugging and annoying problem has arisen. An extremley bitchy annoying girl from my school has decided to follow me on twitter, and is constantly replying to my tweets with silly and infuriating remarks. I’ve decided to block her, and I know she’ll ask me about it tomorrow. How should I respond to her enquiry? I don’t want to come across as bitchy myself, so… don’t really know how I should tackle her. Suggestions?