They can continue using POW (if they manage to coordinate an independent self-destruct mechanism override) however they cannot force the actual users (private individuals and infrastructure providers) to do so.
If miners don't manage to capture a significant fraction of ETH value in their hypothetical ETHPOW chain it won't be worth their effort. Buying a GPU for mining now caries a significant business risk and prices are already declining.
PS: There are billions of dollars already staked in the (currently empty) POS chain.
I do wonder what miners would do with all their hardware and infrastructure tho. Presumably they'll all just move to another cryptocurrency and nothing really changed, etherium got a little cleaner while bitcoin or whatever got a little dirtier.
But I do hope that, somehow, all those miners will just... accept the move to PoS and turn off their mining farms. I just don't see it as extremely likely.