Spreading Euphorbia

I believe this is a type of Euphorbia.

It is pretty much taking over the whole flower bed and beyond, spreading through the root system and seeding where I don't want it.

Looking for a way of control it and remove much of this growth which doesn't involve digging up the other plants. It's a very clay based soil which is very hard when it dries out, so trying to pull it just leaves all the roots behind at the best of times.

Any suggestions very welcome. I'm not averse to using chemicals if need be (but would prefer not).
Thanks