Arrest founders, freeze exchanges and seize their ether or cash proceeds of ether sales recently completed. Subpoena their records and trace ownership to natural persons; if they don’t surrender their ether or pay a fine they go to jail. Trace IP addresses to scare a few more people; rinse and repeat periodically. Bonus points if you do it only for ether, thereby giving people an exit to other cryptocurrencies and allow people to voluntarily surrender ether for a token, capped tax write-off.
The U.S. government has lots of experience thoughtlessly banning things in an overreaction, and with limited exceptions (e.g. the war on drugs) it’s more or less worked.
