You don't need to. Absolutely, if you ban cryptocurrencies, only criminals will have cryptocurrencies. It will, however, add friction - and make it harder to acquire and launder the funds involved.

I think a complete ban on cryptocurrencies is unlikely to succeed, for much the same reasons that the US hasn't banned guns and that the war on drugs is such a shitshow. A punitative tax: 10% of every transaction, for example, would still make cryptocurrencies viable for some extreme schemes, but would make the practice much harder and help establish the "real identity" -> Bitcoin address audit trail. Al Capone was busted on tax evasion, after all.