I worked with a gelding once with the same problem. It may sound strange, but whenever he went too fast and ignored my cue to slow, I collected him into a counter canter. We would canter a few circles, making him work hard using leg and rein. After three or so circles (you'll feel when it's the right time) I loosened the rein and took my leg off him. Counter canter is a lot harder for the horse when it's fast, so he usually got the point and slowed down. After a few strides, keeping him at a lope, I changed directions back to the correct lead circle, and collected him with very gentle cues, and let loose again. After two circles at the slow lope, I'd break to a walk, collect him right away for a few strides, then loosen up rein and leg and walk around the arena a few times, total break for him before starting again.