it sounds stupid, but what you need to do is ignore it. you just need to keep doing what you are doing and ride through it. do not stop hold on and continue. Your horse will eventually relize that bucking isn't getting him anywhere and he wont do it anymore. it just takes time.
My equitation horse is an insane bucker (it's just him, he has no problems what so ever as we have had everything checked) and it took me a while to learn to ride him and not allow him to buck during our rides. I would advise you check out your horse to see if it has any pain anywhere which may be causing it to buck, and see how your saddle fits. You have to learn to sit back, put weight in your heels, putting leg on and lifting your hands when your horse does so. Pushing them forward will make it harder for them to drop their head and buck. When they buck, make them work harder. Learn what your horse dislikes, back them up around the whole ring, do small circles and force them to do work, then they will learn when they buck they will be forced to work.
Hold the head up with the reins. If its just one buck, sit deep with lots of pressure on the stirrups to maintain balance. It its multiple buck after each other, a two-point seat can help to let the horse buck under you without losing balance and being jostled around too much. Also, figure out why the horse bucks and see if you can solve the problem that way.