You address this by taxing at the fiat on/off ramps. The ones with access to the deepest liquidity also happen to operate in jurisdictions that have functioning governments that are amenable to passing taxes on PoW. And if that doesn't work, you impose sanctions and tariffs on the countries that permit limitless PoW mining, up to and including black-holing their IP blocks.
It can be done. We banned CFCs after all.