I’ve thought through this over the last few days and I don’t think it’s viable unless all countries enact the ban at the same time. Imagine country A bans trade or mining of PoW coins - that puts the citizens of country A at a disadvantage to citizens of other countries, and encourages holders of PoW coins in country A to exfiltrate wealth and capital from country A.

You end up needing to solve the same type of problem that PoW solves in order to enact a PoW ban simultaneously across all countries - how can all parties trust that every other party is being honest and will follow through (Byzantine Generals problem)?