i agreed with "holiday". it's illegal to deduct the maid's salary and which is not a good way. you will be very risky later on if she will sue you. no matter she will win or not, you will be in trouble.
actually, for giving a warning letter is just to scare her. if she insist to be like that, the warning letter will not be useful anymore because by law she has the right to do so.
for my opinion, try to ask your agent to consul her and see if she will improve. but if not, then i think this is her character, she can't change because she is the kind of person weak in self control in someway (if i am not wrong). you will only have to either accept it or to terminate her.
even if she will improve at the beginning, you still have to remind her from time to time about every 1-2 months if you want to keep her to work with you. some maids are like that....