If it's growing on the ground, the only way is to pull it up. Weedkillers don't get rid of it. This is my task in the garden this weekend and my arms are hurting already but there's so much of it and if I wait it'll take over the garden completely.
I cut it down below the soil and then painted engine-oil mixed with BWK (Brush-Wood Killer) over it, then covered it with a piece of black polythene bag which I then covered with soil. that was about 6 years ago, maybe longer, and have`nt had any regrowth.