I think you are lucky to have an Arab that has a big canter stride. Many of the arabs I have seen dont'. They often have a shorter but quicker stride and can sometimes make the three beat canter become almost a two beat "pronk" like a gazelle.
As far as I know, slowing a canter can be done from the rider sitting back a bit, really engaging the core muscles and kind of "holding" the horse back, and when the horse slows you have to be quick to reward them by loosening and letting them go. Which of course, means they will speed up again, you tighten your core, kind of firm up your body and stop moving with them so smoothly, theynslow, you release and reward , and on and on . Eventually, it will take the minutest tightening of your core to remind them to slow and if you add a little closing of your hand on the outside rein, well, it'a a basic half halt.