You can ban a lot of things in life. Theft. Drugs. Crime in general. You may not understand how the OTC market works in jurisdictions like China that do indee

You can ban a lot of things in life. Theft. Drugs. Crime in general. You may not understand how the OTC market works in jurisdictions like China that do indeed ban cryptocurrency exchanges and prevent fiat<->cryptocurrency exchanges. A fist full of RMB can be swapped for USDT, which can be swapped for BTC from OTC brokers. What you are proposing can only be stopped by breaking computers, breaking arms/legs and breaking economies. Violence is never the solution to problems.