Funny you should ask as i asked someone in my allotment today. For now pull out each one slowly so you get the full plant. He then suggested, in the autumn, to add a low grade weed killer to it, then mulch over winter.

You rarely get the full plant....any tiny white stem left behind produces another plant. I have this problem on my allotment. As Redfingers suggests......the autumn is the time to spray effectively... when the plants die back for the winter they are more like to draw the weedkiller down and actually kill the plant.....You need a lot of patience between now and then!!!