All they need to do is make it illegal to complete an enforceable contract with crypto. In other words, you can sit and stare at your coinbase account all day, but you can't ever legally get hard currency to spend it, and you can never pay for services or products with it. All of this is trivial for world governments.

And the underground economy is a rounding error, if BTC is relegated to the dark web, it will lose 99% of its value.