I currently have a mare that is doing much the same thing -- not to throw me, just a "look what I can do now, mom" kind of thing. We have not progressed to a gallop yet though. In my case, I push her forward more to avoid a succession of bucks, and then bring her back down and do another loop at a canter to tell her, "oh yeah? Well, look what I can do!" Works really well in a foot of snow!
If your girl can kick out at the gallop, she can go faster too -- she is NOT at top speed when she is in mid-buck/kick. I would practice, practice, practice that gallop. Feel for that buck-to-come and urge her faster before she gets a chance to buck.
I don't think I would smack my horse -- I'm not sure she would associate that with the bucking being bad. But then, I basically don't hit my horse at all when I'm riding. Not for any real reason, other than it just doesn't occur to me. I definitely wouldn't spin her either -- not something I want my horse to do. I would turn her if needed to throw her balance off if she went into a fit, but not just for a single buck.
This assumes that you have no problems with her at slower gaits. If you do, then I would start at the slowest gait with problems and move up from there.