In Ontario you need a Pesticide License to purchase or use Tordon 101. Or you will have to hire a company with an applicators license to spray for you, although you would have to check your local and provincial laws as it may not be usable in your area.
I've always had good results keeping Manitoba's down by drilling the stump once the tree is gone and pouring powdered milk in the holes. (old trick from my grandfather)
Tordon is a Herbicide that works well on killing Trees, bushes, and just about anything that needs killin'.
Sounds like it is restricted in your area though.
Possibly the "Ready to use" formulation would be available? Look for Tordon RTU maybe?
Other options that will work, in case ya gotta find one that is allowable.
2-4-D - Weedar64, or just about any formulation with 20+ percent 2-4-D.
Crossbow- Uses 2-4-D and some other evil stuff. Wont kill grass, but it will kill everything else. Don't get it on on near desireables, and be wary of root to root cross contamination.
Roundup/glyphosate. Will work, but you might have to hit everything 2-3 times. Use it straight and don't dillute.