Sounds like she is just testing you. You took her head on, by not letting her spin when mounted, and she reacted with a blatant "NO!"
Which is probably the reason she keeps repeating the behavior. She needs to be taught who is more dominant, you or her.
My mare tried this with me, once, much like Jessabel stated. She was much younger, and caught me right in the knee. She had never done it before, but did it when she was excited. Unfortunately for her, she was in a roundpen. I literally cracked her on the a$$ with my whip, and made her run til she was really breathing hard. Never had that problem again.
I rarely ever hit my horses, but when they start something, I finish it. That mare had at least 1000 pounds on me, and I knew I would not hurt her by hitting her when she was the one who was disrespectful.
Whenever a horse tries to walk as I am mounting, I put my foot in the stirrup, and "hop" with them. Yes, I hop on one foot. lol.
And then I circle them by pulling on the rein closest to me, bringing their head into me. They circle til they decide to stand still. I keep doing it until they get tired of circling. Just a thought if you don't like lunging.
Oh and PS, my mare was fine with me right after the incident. In fact I found the harder I was on her, as far as manners, the more she liked me. For a horse, having a leader is much more comforting than having a friend.