The horse I jump on always does it.
Ok, well, sometimes he trots all the way, but usually its just on the edge of canter xD
You just need to hold him back, but remember to be gentle with your hands all the same. Don't harden your hands!
I know you might say:
"What?! Have soft hands but still hold him back?!"
But yes, that's what you must do xD
It helps putting a pole in front of the jump and then its ok if the horse makes a "half-canter" when crossing the pole.
I changed trainer and now I always gallop to the jump.
That might be easier. But its good to train the poles too!