Removing Comfrey

I'm trying to remove a patch of common comfrey because I'm a cretin and planted it in the middle of the veg garden where it self seeds everywhere! 
So far I've probably snapped 95% of the tap roots at the depth of a fork because my subsoil is boulders in clay, perfect for burying and hanging on to long roots  What chance it won't all just grow back?
On the plus side the soil is lovely underneath