I start and stop. Meaning I'll canter then stop. If they don't stop when I ask, I back them up. That usually slows my horse down.
Give your horse a small squeeze with your reins. Lean back and really sit into the saddle. If it doesnt work try taking you horse in some circles. Hope this helps!
Do some circles and stop the horse carefully. Do not bomb the horse behind -its make it a punishment. Clap your horse when it's stop.