Means you need to fix something earlier in your training. If the horse won't one-rein at a canter, go back to the trot. Flex more, disengage the hindquarters more, and have feel with your hands - give and release. I'd get a full-cheek snaffle sweet iron. Kimberwickes are the english equivalent of a curb bit and will only solve the problem until he realizes he can take advantage of it again. Go back to basics, you're missing something in the foundations. The one-rein stop is not an end-game for stopping, stopping without the reins is. He should know by the movement of your body that you're going to ask for a stop, and the one-rein is reinforcement. If he doesn't, you need to back him up.