I used to have a pony that bucked. Not big bucks but I couldn't for the life of me ride him out (bear in mind I have ridden out HUGE heels-above-ears bucks, and that was BEFORE said pony).
He would buck, I would whack him hard on the bum with a dressage whip. He would invariably buck again, so I would whack him again. Rinse and repeat until he stopped bucking and settled. Worked darn well with the brat.
I've also dealt with another bucker, and what worked for HIM was the Monty Roberts buckstopper (should be detailed on his website). Brilliant piece of equipment which I still have in my tack shed waiting for the day I might need it again... and I very nearly went there. Before I got Magic I was looking at a STUNNING gelding who bucked something chronic, and seriously considering, because I knew I'd solved the same issue before with two different horses, and I was specifically looking for a project. The buckstopper is sitting in my tack shed in case I do need it for Magic, though I don't think I will. She rears, but I have yet to see her buck.