I use vinegar on mine. I have a continuous problem with them as they seed over my fence from a common.
It is not mown as frequently as it used to be in an effort to encourage wildlife (which is a good thing) but that does mean the clocks have time to develop and they all waft my way on a windy day.
I also prevent any I have from seeding just by picking. At least that way I know I can eventually fight just the one and will not face an army of babies....prevention is better than cure...as they say.