There are 2 methods I usually use to slow a horse down at the lope...after they are semi-balanced and comfortable with the lope itself. To get them balanced and comfortable, I just do a lot of it. I'll get on a green horse and take them out in the country and lope them off and on for a few miles, that will get them balanced on the straight, then I come back and work circles at home the following day. I follow an old saying that hasn't failed me yet when working a horse in circles... "Lope them until their head drops". Basically, you lope them until they relax and flatten out like a good saddle horse should, it may take 10 minutes, it may take 2 hours, but it will come and both you and the horse will be better for it when it does. After they have begun to relax and flatten at the lope, then you'll know that they are comfortable and balanced and ready for one of the following methods.
1) Make the circle smaller. Start out with big circles and when he starts to blast around, bend him to the inside and make your circles smaller. Keep making them smaller and smaller until he gets to the speed you want and relaxes, then slowly start letting him enlarge the circle, more like a spiral out to the bigger circle. Don't shrink or enlarge the circle quickly, just a nice, steady spiral in and out and don't let him break gait. If the circle is small enough that he feels like he's about to break down to a trot, just keep it that size and keep him pushed just enough to keep him loping even if he's still rushing a bit. Keep the circle small and he'll get tired and rate himself in a minute (though you may be more tired than he is by the time that's done LOL). So long as he keeps the slower pace, let him keep going to bigger and bigger circles. If he starts to speed up again, take him back down into the smaller circle.
2) Get him working with his brakes on. This method will have to be started at the slower gaits first until both you and he can get the feel for it and get the timing down. Start at the walk and when he's good and consistent at the walk, work up to the trot and do the same thing until he's consistent there, then you can move up to the lope and work on it there.
Ask for the lope and then let him lope for 4 or 5 strides, then ask for the stop, back him up a couple of steps and then hold him there for a minute until he's soft and flexed nicely. Let him down and ask for the lope again, let him go for 4 or 5 strides, ask for the stop, back up, hold, let him out, ask for the lope, etc, etc, etc. Do that over and over and over and over until you feel him begin to look for that stop, he'll feel like he's hesitating with every stride and that will slow him down. After he starts to "move with his brakes on", then start letting him lope and relax at the lope so long as he keeps the slower pace. If you feel him start to speed up or string out, stop him, back up, and hold again. If you work on this consistently, he'll end up with a super nice little flat, collected lope that you can ride all day long. That's how my Dad used to train all of his old western pleasure horses.
I'll normally alternate between these 2 methods and sometimes I'll even mix them together, it gives a bit of variety and keeps the horse from anticipating anything.
Here is an example of how I might spend some time on a young horse just working on their lope and their handle:
Lope a few big circles then start to spiral down to a relatively small circle, start to spiral bigger, stop, back up, hold, lope off, spiral back down, stop, back up, rollback into a lope the other way, stop, back, hold, lope, stop, back, hold, lope, spiral in, spiral out, lope a few big circles, stop, back, rollback into a walk, let them walk a few circles, stop, back, hold, lope off, stop, back, hold, trot a few circles, stop, rollback into a lope, etc, etc, etc.