This may be a radical take, but I think nations should introduce some unprecedented legislation: ban trade of proof-of-work cryptocurrencies.
Don't ban their trade because they make poor financial products, either because of rampant fraud or criminal activity. That's a different argument and requires different approaches. Ban their trade because global society shouldn't accept rampant incentives to literally burn up energy [1] to make financial products. Especially because proof-of-work simply just isn't necessary to have cryptocurrency.
Banning their trade won't categorically stop PoW cryptocurrencies. What it should do is completely tank their value and get the world to move on to less destructive coins.
I don't think there's any precedent for banning classes of financial products for environmental reasons, but it's time to create one.
[1]: In addition to environmental reasons, there's probably also economic ones. Mining burns through other scarce resources such as chip production capacity, although the true impact there is unclear.