I did completely clear one allotment of mares tails but its coming back in. My other plot on a different site is riddled with it but I cannot honestly say that it has a detrimental effect on any vegetables that I grow. I just keep nicking it out either with a kitchen knife, a trowel or just hoeing. I try to cut it as deep as I can get without disturbing the soil too much. It does help tremendously if the area is dug over, removing as much as possible in the process.
Pete