What are you like as a rider? Brave?
You can go two ways about a problem like this - which, by the way, is not 'obsessive' and set in stone, if it was only one ride and he's been out of work for a little while he's probably just feeling a bit full of himself.
You can go for the above option, of bringing him back and not letting him canter until he settles in trot, or, if you're gutsy enough to do it, LET him canter but drive him as hard as you can, so that he is having to work twice as hard as if he was trotting. Drive him around the circle for a few laps, then ask him to come back to trot. If he doesn't respond immediately, drive him back up again.
Some horses use canter as an evasion when the trot work starts getting hard. My gelding used to do it as an evasion in trot as soon as I'd start picking up some contact and asking him to come through the rein. He'd go ' well thats a bit hard' and canter instead. So I'd drive him on, make him get a bit of a puff going, then ask him to trot. The first time it took me 5 canters to get him to stop using it as an evasion. After that, as soon as he went to canter as an evasion, I'd put my leg on, canter a few strides, come back to trot and he'd be fine.
He has not done it for over a month now, he'll think about it, do one stride, and then go back to a good trot.