_If_ I wanted do this (and was a government): - Prohibit trading of cryptocurrencies, punishable by several years in prison if caught. Structure it so that you can personally go to prison even if you did it as part of a business. This should 'discourage' casual investors. - Pass a law stating banks can lose their banking license if they allow transactions from/to crypto exchanges.

Basically, declare cryptocurrencies to be similar to hard drugs or something and follow the same playbook. Obviously the real hardcore crypto lovers will still keep using it, and it will also not deter criminals who were outside of the 'regular' system anyway. But if you have a functioning law enforcement system like most western countries, you could certainly keep cryptocurrencies from playing any large role in your economy.