It'd have to be an international effort as there are big mining operations are in Russia, Switzerland, China, Iceland as well as the US.
Being legal means that you can run big mining operations, so you could clamp down on those and slow mining. That would not stop it, though.
Being legal means that it can be used to trade goods and services, and you could clamp down on that and harm its value as a currency.
And being legal means that legal businesses can exchange it for other currencies, so clamping down on that harms its liquidity.
Even if you can make it broadly illegal across the globe, it's hard to see how effective that would be. Illegality has made anything else on the black market go away, after all, and the whole point of a crypto-currency is to thrive despite government suppression.
Being legal means that you can run big mining operations, so you could clamp down on those and slow mining. That would not stop it, though.
Being legal means that it can be used to trade goods and services, and you could clamp down on that and harm its value as a currency.
And being legal means that legal businesses can exchange it for other currencies, so clamping down on that harms its liquidity.
Even if you can make it broadly illegal across the globe, it's hard to see how effective that would be. Illegality has made anything else on the black market go away, after all, and the whole point of a crypto-currency is to thrive despite government suppression.