I'm afraid you have to dig out as much as you can each year, it leaves tiny creamy bulbs in the soil which are almost impossible to get all of them out. Or if nothing else is growing nearby/in between you could use a glyphosate weedkiller. However, I have found that the bees like them as early food. Then I dig out as many as I can manage! It's a long term problem, but they die back early on so you won't see them for long!