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I also agree with the insanity that is the energy consumption associated with bitcoin mining.
If CO2 emission, and its negative externality, is specifically the problem, then you can advocate that all production of non-essential/luxury items use non-CO2 emitting energy.
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 enjoy spending your resources on.
It reminds me of the disgusting and flagrantly ridiculous thread from Stephen Diehl that was on HN recently, more comments here:
The amount of energy used by bitcoin is many zeros larger than any of the examples listed and is used by a group of people many zeros smaller than any of the demographics mentioned. This is a pretty easy case to argue objectively, particularly because there are alternatives to proof of work, but they aren't as sexy as running a server farm full of FPGAs, GPUs, and ASICs.
If you had carbon taxes in place, this "environmentally unfriendly" activity would go away.