As I wrote a couple weeks ago:
If you want to take a Puritanical attitude, any luxury product/service can be argued to be wasting resources that could be expended on essentials. In other words, its alleged lack of utility doesn't provide a justification for destroying or banning it. It's up to others to decide how to spend their resources.
If CO2 emission, and its negative externality, is specifically the problem, then the principled position is to advocate that production of all non-essential/luxury products/services use non-CO2 emitting energy inputs.
Unless the measure is agnostic to what category of non-essential activity it targets, you'd just be singling out cryptocurrency, because it's not a non-essential that you personally believe is worthwhile enough to pollute the environment in order to produce, unlike say gaming, and the massive amounts of energy that gaming GPUs use.