I've been through this, it isn't fun, but you can win. I found Revive to be stressfully on the corals for repeated dipping. Have a read
here, it works well, and the corals don't even react to the dip if done correctly. Unfortunately it's only available in The States. Perhaps you could test some infested frags with an insecticide that is available in Canada. B-Cyfluthrin is the ingredient that kills the flatworms.