I don't know what legal avenue will be taken should the federal government pursue a ban, but that's probably not it. As you point out, that would have unpleasant slippery slope consequences. At the same time, it's pretty obvious that something needs to change.

As-is, currency mining is an effective means to leverage low electricity prices around the world and is very-often more lucrative than producing things we actually use to improve our standard of living. Suppose you impose carbon taxes, you'll (often) be preferentially biasing towards mining over e.g., aluminum or glass manufacturing, driving physical goods shortages. And even if the entire grid were renewables, you haven't really solved the "it's more profitable to make digital goods than physical ones" problem.

So you search for regulation that is biased against the digital currency but not against uses of energy. Maybe a digital currency transaction fee, like a sales tax, that puts a ton of drag and downward pressure on the currency? It could be huge, and it could be modulated by the energy use that any given currency represents or the CPI. Of course you have the cooperation problem; taxing the life out of a currency in one country is only modestly useful.

It's a thorny problem.