My gelding used to do this, and I would just give him a big smack on the butt as soon as he started backing, but for some reason I don't think that would work with your horse.
Does he do it on the ground or while you're riding him or both?
I would make him do a 180 as soon as he backs up. As soon as he completes the spin, stop him and give him a chance to hold still. It might distract him enough to keep him from continuing to back up. If he keeps backing up, make him do 4 or 5 spins and then see if he'll quit.
I like the other girl's idea too. Back him up about 30 feet or until he's sick to death of walking backwards. That's the same way I trained my other gelding to stop spinning and trying to go back home. Every time he spun, I'd spin him in circles about 10 times. Eventually he learned that if he tried to spin, he'd be forced to spin a LOT, so he stopped trying.
Last edited by CloudsMystique; 08-13-2009 at 06:02 PM.