I fired her. Again!

"C, I dunt think I am suitable for your jobs. Will invoice you, 30 days payment terms. Tks."

I cannot believe she is still very capable of driving me UP the wall. Over the phone, we have already agreed upon the specs and scope of work for that specific rate, and she simple disregard all that insist that I do more than what we agreed upon. So very her.

Me: I cant do it.
M: we agreed that I only do this this this, for this rate. I m sorry I will not do more.
C: theres also CD face design!
C: but u r finishing soon. then wat u do tomorrow?
M: I still have the remaining stuffs which I have agreed to deliver, I will do those.
C: u need so much time to do those meh?
C: it is understood wat - Cover design is also together with back cover design.
M: C! arms are arms. legs are legs. I only agree to do cover, I never agree to do back cover. sorry.
M: if like that, then tomorrow I wont come in liao
C: but, there! client also wrote here - cover PLUS back cover. (showing me the client's brief)
M: thats between u and client. nothing to do w me.
M: U still want me to do CD face...... wah lau.
M: if u want me to do back cover. Can. Take out one item, and i will do it for you in place of that.
C: Cannot la!!!!!
M: I cant do it.
C: ok loh then u do whatever u say.
She bloody tossed the brief right in front of me on my desk. She is out of her screwed up mind.

Tell me, how to deal w such ah soh?