Keep digging it out unless you do want to resort to a strong weedkiller.

But personally keep digging.

Good luck.

Digging and removing every last bit of root is the best answere. If it is growing through a hedge and you can't dig then a glyphosphate weedkiller like 'Roundup'. With this be careful not to get it on the foliage of any plant that you want to keep or the Roundup will kill it, also. If the ground elder is growing in a gravel driveway that you do not want to dig up and will not want to plant anything in, then a total weedkiller such as Sodium Chlorate but remember, this kills eveything and is persisten in the soil.

Unfortunately sodium chlorate is no longer available to amateur gardeners, but aggree that it was the best weedkiller to get rid of almost anything

Dig, dig and dig again. We get it growing under the fence from next doors garden. Just when you think you've seen the last of it, up it pops again. Unfortunately for me, it comes up between plants so it's a never ending job. I've noticed it seems to appear especially after heavy rain. Good luck!

Hmm, well this is one time I'm all in favour of glyphosate I'm afraid. I had a real problem with ground elder in two adjacent gardens, but worked out a method which worked in a season. You need it to be growing strongly and the soil must be damp, with no rain forecast (so late spring at the earliest). Then you spray every bit of foliage with the glyphosate solution. Wait about 8-10 days and spray again, wait another week and then dig it out. The glyphosate might look as if its killed it, but it won't have - what it will have done is kill off the hair like fine roots that are inevitably left behind when you do dig it out, so if you follow this method, when you do dig out the main roots and plants, it's all gone. It never ever came back, and I still maintain both those gardens now, ten years later...
Were I you, I'd stop digging now, no point, it'll all be back next spring... but its not me doing it, it's you, lol!