Is she saddled? If she is, you are just teaching her to buck.
If she is not saddled, spank her butt and put MORE pressure on her. She needs to be 'running for her life' after misbehaving. Then, do not make her go faster and harder for very long, but make sure you NEVER back off when she is misbehaving. Always quit when she is doing the right thing in the right way.
You do not have to 'back off' at the wrong time very often before you have created a monster that knows as soon as she blows up or misbehaves, she wins and YOU quit.
I agree 100% with Cherie with one caveat. You didn't say much about what you are doing when the horse starts with her antics, or whether the horse is normally stalled or kept in a pasture. I have no problem with a young horse showing off and blowing off a little energy by playing a bit when I first cut them loose in a pen or corral, particularly one normally kept in a stall and fed grain
, and I might let a 3 year-old play a little, but as soon as we start getting down to business and I start giving commands I expect it to stop. From the short description you gave, it sounds like that's what she is doing, rather than being aggressive or disobedient. Still, as soon as it's time to get to work, it's time to quit playing.
In my experience (which is admittedly a lot less than Cherie's), just like a kid, as a horse matures it tends to grow out of the need to blow off energy as much, but sometimes letting the horse play a little and blow off a bit of pent-up energy actually helps it focus better once the training starts. There needs to be a clear delineation between play time and work time, though, so she knows when "playtime" is over.