They can prosecute transactions with unregistered keys as money laundering. They may not have the resources to get everybody but they can collect a few scalps pour encourager les autres.
Yep, they can also prosecute merchants who accept payments that don't go through "approved" middlemen (banks, etc), or even ban/regulate crypto that allows users to make their own keys.
At one point the US gov was building their own crypto currency, and I'm sure it wasn't because they felt the existing options weren't private enough from oversight.
Stopping cryptocurrency is as hard as stopping end to end encrypted messaging or banning porn.
You either allow access to the open internet or you do not.