Oh wow Chevy, I've never had that problem. I have had some that didn't want to swallow but never had one that wouldn't let me cum in her mouth.
You could try withholding sex from her but that never works when us men do that....lol.
Nah, really, just talk to her. Tell her you just want to cum in her mouth and then she can spit it out if she wants to.
I have a good friend that told me the same thing about his wife. I made it a mission to shoot off in her mouth at least one time.... lol.
One night at a party at his house it got really late. It got late and everybody had left except for my wife and me. My buddy passed out while we were watching tv and my wife passed out/fell asleep too. His wife and I went outside for a smoke. We had had a lot to drink...lol. I told her I had heard she didn't like for a man to shoot off in her mouth. She said where'd you hear that? .. We joked and laughed for awhile and I told her I'd love to shoot my load in her sexy little mouth.
I kinda pushed her down on her knees, I opened my pants and she just lended over and sucked my cock right into her hot mouth. Oh man, she's a really good little cocksucker. That and the fact that we were sneaking around with our spouses right inside the house excited the hell out of me and I didn't last long. The first spurt gagged her a little but she kept sucking and she swallowed all of my load. She said she didn't mind a man shooting off in her mouth and that her hubby had just never asked....lol.
I did tell him later to just ask her and see what happened. He's never told me if he's asked her or not.
Good luck