Insofar as the concern is about carbon impacts related to cryptomining (which is not an unfounded concern), there are several policy interventions which could either help to de-link crypto-supply and carbon emissions and/or reduce demand for cryptocurrency, thereby reducing the price and dis-incentivizing supply:
Very Direct Method:
Criminalize cryptocurrency - Even if enforcing such a law would be difficult-to-impossible, there is a natural deterrent effect in adding the risk of criminal prosecution to transactions. The Streisand Effect (making something popular by denouncing it) is curtailed here by the fact that cryptocurrencies are a speculator's product, the people who have the strongest impact on price are also the most likely to be detected and prosecuted. Even widespread black-market use wouldn't be enough to hold cryptocurrencies at high prices.
Crackdowns on sites like Coinbase, etc. are demonstrably easier (at least for US authorities) and would follow the model the UIGEA enabled for a crackdown of online gambling, but especially of online poker sites. This would effectively limit the broad-base public access to cryptocurrency trading, and push it back into a niche fiscal tool, again lowering demand.
Moderately Direct Method: Tax cryptocurrency transactions differentially. This could only be done to businesses, who have to maintain good books and could be audited to ensure compliance. This directly raises the transaction cost of using crypto as a currency, and thus fewer merchants will want to use it, relative prices will be higher for products bought with crypto, and as a result demand for the currency will decline as its utility erodes. It's not yet vogue among large corporations to accept crypto, so this is more a measure to prevent things from getting worse.
Indirect Method: Any effort to decarbonize electricity helps to de-link cryptomining and carbon emissions. Carbon taxes are the most effective means here, as it will ensure that cryptominers select for lower carbon sources of electricity (or produce their own), but renewable energy mandates and other policy tools to push renewable or nuclear power sources will have similar effects.