I am having a unwanted relationship with my stepmother?

She was irresistable for a while. I only started sleeping with her because she gave me a sob story that my dad has been hitting her. That turned out a lie. I told her that i was disgusted and we had to end it but she said if I ended it she would say to my dad that i forced myself on her. I was shocked that she stooped so low. We had a couple more times and have tried to avoid her ever since. What can I do to get her off my back?