Hormonal imbalance?

Our three-year-old dog was spayed a year ago (after a litter of pups), but, and there is no easy way to say this, still constantly tries to hump our male dog! He is having none of it (bit of a wimp) and she is so dominant that he sometimes squeals where she grabs him so tight she hurts him! She is a very 'tough', robust girl with a happy, playful demeanor but sadly we constantly have to pull her off of him and stop her playing as she is hurting him. What can we do with our bully? In every other way the two dogs (same breed) get along well. What is happening with her hormones? We thought spaying would help correct this.
Paul Andrew, by email