At best they can ask the public to please turn over their private keys, which will go about as well as efforts to stop piracy.
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.
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.
Well sure, but if you want to actually use it you need the whole keypair. Unless you're really good with you elliptic curve abacus you're going to need a computer for that l.