first of all try to determine whether it's really the horse that's rushing, or maybe you ...
sometimes it's a vicious circle, you're expecting your horse to rush so you tense up and that makes him run even more, same goes for trying to hold them back. If you have the opportunity jumping different horses helps to find out.
Most often horses that rush their fences aren't strong and conditioned enough or don't feel confident enough about jumping the fence so they try to compensate with speed.
Good excercises to help them are pole work in trot and canter, lots of grid work. With the grids you can try to decrease the distances slightly instead of making them all even, so they have to pay attention and get slower. Canter poles in front and after a cross or vertical have the same effect.