I'm sure all of these examples (encryption, guns, abortion, crypto currencies) are considered by some people to be that extreme situational outlier, and needs to be banned yesterday.

Mine is proof-of-waste crypto currencies such as Bitcoin, or Ethereum before the PoS merge. Too much CO2 for too little gain.

(There's also the Ponzi aspect, but I don't think we need new laws to ban Ponzi schemes: if a crypto currency turns out to be a Ponzi scheme, just sue them for making a Ponzi scheme.)