If you're in Ontario, you may not have access to the chemicals we have in the States. If you do, two sequential applications of broadleaf weed killer, about 2 weeks apart, usually does the trick.
If not, you can try the "20 mule team" method. This recipe if from the Univ of Minnesota:
Dissolve 10 oz. Twenty Mule Team Borax in 4 oz. (1/2 cup) warm water. Dilute in 2.5 gallons of water.This will cover 1,000 square feet. If you have a smaller area to treat, cut the "recipe" accordingly.
apply evenly to the treated area in spring, and be sure to follow the recipe carefully. It doesn't have a lot of room for error -- you can poison your soil for ALL plants if you apply too much. That's why i think the herbicides are a little safer in this case. But the borax works too. You just cannot apply it more than twice to the same area.