How do you deal with emotional manipulators?

I broke up with this girl a record of 8 times..I have tried to be nice going about it.. I know she has a fiance and I understood our relationship was based on pleasure. Yet she is always crying that I am leaving and does not want to let me go..I know it is all Bull$hit..since she still carries a diamond ring with her..I do not understand why she feels she will keep me by manipulating my emotions by crying all the time...should i just not give a damn and tell her how it is or totally just use her until she gets preggers?