Keep it cut back so the roots will die because there are no leaves to supply the necessary nutrients and dig the roots out of the soil.

This is a nasty beast. I spent a good portion of the day yesterday digging out some huge tuber like roots. I wish I knew of a good weed killer that could kill to the root!

It does seem like a weed but it has medicinal qualities. See if you can find a local herbalism person to come and dig or buy the roots from you after you've dug and dried them.