My point is that you won't fix ransomware by banning cryptocurrencies. A global cryptocurrency ban is very difficult to envisage, and will do very little to stop ransomwares. Offshore exchanges will still be a thing (it's already very common for US citizens to use offshore exchanges via VPNs in a regular basis), local p2p markets will still be a thing. But you now also banned honest actors and lost your main tool to regulate strictly what is happening domestically (i.e exchanges under your jurisdiction).