My big girl doesn't bunny buck when she bucks. They're HUGE. She's only gotten me off once but in my eyes once is too many times. She's always genuinely terrified when she does have a go at it but she's in the process of learning that bucking is unacceptable under ANY circumstance. [edit; she's very green, too - 2 1/2 and with under 30 rides. Some bucking is pretty normal with most greenies, especially if they didn't buck during the breaking process]
Simply put, you haul their head up, push them forward, and ride them through it. If you have to, bridge your reins, because then the horse is only fighting against itself for head down.
The best way of stopping the buck is to feel it coming and bridge your reins BEFORE the horse can get its head down. Keeping the head up removes all the power out of the buck [not that a bunny buck HAS any power!] and then it becomes much easier to ride out.
One of the girls I ride with has a pony that bucks her off with monotonous regularity... someone really needs to teach her how to handle it when he does buck or he's never going to stop...