The basic idea of the economy is people can earn the right to energy, and then direct it to a goal of their choosing. The government making a value judgement about what type of commercial activity is worthwhile is extremely risky. Countries with innovative financial and credit systems have historically steamrolled those that don't have such innovations. We still don't understand the nature of crypto.
Edit: I suppose, if you want to subsidize all uses of electricity except for cryptocurrency (and a few other areas of your choice), you could simplify this process by only applying the externality tax to miners (etc). But this should be limited to the externality, not arbitrarily high.