I use glyphosate, doctored with a few drops of washing up liquid as a wetting agent. I use a hand spray (an old glass cleaner handgun) and gently squeeze a few droplets down the V in the leaves, so it runs down to the base. If you have a lot and they are mixed in with more desirable plants you could use a rubber glove on your right hand, dip your finger and thumb into a bowl containing glyphosate solution, shake droplets off into the bowl to avoid accidental dripping, and then rub finger and thumb along the leaves to smear them. That worked a treat for me on MIL's garden a few years ago. It also got rid of ground elder in her lawn!