Do what everyone else is saying but if that doesn't work plant one hand on his/her neck and take the other rein and pull it back to your hip.
If he/she isn't listening to pulling back then you can try wiggling your hands back and saying woah walk in a very calm tone but make sure that they can hear you. Sit back on the saddle and they should start to slow down
Try to circle until you are in control. Open the inside rein down to your knee if he/she is difficult or if he/she is making hard to go into circle.
Use your seat when slowing down. Your horse should feel you ever so slightly lean back and pull not down but at an angle that is in line with the rein and bit to provide you with the most effective and efficient response. When using your aids with co-aids your ask becomes more assertive.
Hi, I find half halting very effective, but I would also (in conjunction with half-halts) put weight into my seat and tighten my core muscles(I personally try to use more than one aid at a time) . If that fails putting them on a circle may also help. Best of luck!
My horse does the same thing sometimes. I recommend half-halting when your horse picks up speed in the canter by pulling on the outside reign.