Bruce force keep digging it out.

Persistence, effort and dedication - and glyphosphate! Spray or drench the leaves while they're growing strongly, and the soil is not bone dry, but no rain is expected for a few hours. Leave for a fortnight and repeat the treatment, leave for another week to a fortnight. Now dig the area, removing each plant with its roots as you go. Just digging isn't very effective because ground elder has very fine, hairlike roots as well as more normal thicker roots - the glyphosphate seems to kill off the finer roots. If it looks like its all died after the glyphosphate treatment, don't be fooled - you still need to dig it out.