I’d do it slowly by tainting certain known addresses, money laundering, tax evasion, terrorism, scams etc. It splits the world into tainted coin and untainted, all regulated exchanges will be unable to accept tainted coin. Then expand what gets tainted to include coin with unknown provenance. Also you can get tainted if you send to a tainted address so you have to be careful who you send it to. Then when the details of who is on both sides of the transaction are known I would tax it. Anyone who doesn’t pay gets tainted.

I would also secretly run tumbling services....

After running a publicly massive campaign against bitcoin et al, I would allow conversion to a fed coin for anyone who can prove the coins were legitimately acquired. The conversion rate would be set favorably to the govt. Anyone who complains gets investigated.