Lots of half-halts and a heavier seat, don’t fall into the urge to stand up when pulling back, will only cause them to go faster, so sit down heavy and half-halt. He/she should slow down, and then reward them. If it has a tendency to go back into trot straight away after a half halt, follow up the half-halt with leg so your saying “don’t go too fast, but don’t go too slow”.
Also if you have a school practice cantering on a 20 metre circle so you have more control and ask on the corner nearest to the fence instead of towards the empty space in the school (less temptation to gallop)
Hope this helps!