If you are sure
the horse is NOT bucking because of pain, but is bucking because of irritation or spoiled[ness?], the first trick is to KEEP THEIR HEAD UP so they can't buck. Horses have to lower their heads to buck, so if you keep their heads UP, they shouldn't be able to. You can try [if you're desperate] useing an anti-grazing device, or attaching another pair of reins to the bit and looping them to the saddle horn, but I wouldn't reccommend it. Mostly I'de just work on keeping their head up with your own strength, being able to ride it out, and also being able to think quick enought to smack them on the butt RIGHT after they do it. When you smack them, turn them in a circle [one rein stop]. A circle is punishment--most people won't agree with that but would YOU like to walk in a circle all day? I wouldn't! Also, when you turn them in a tight circle, they can't do anything but turn--no bucking, no rearing, no galloping off, etc. Just act like nothing is going to happen. Be calm. Ride it out, but punish accordingly.