First thing that comes to mind off the top of my head as a US-Govt option here would be something like: bail out US people/companies of bitcoin holdings in USD in conjunction with banning bitcoin in the US going forward. So that would be quite the string of events at that point for non-US bitcoin holders: first a crash that caused all these US bitcoin holders to go screaming to the government for help. Then the overnight removal of a huge chunk of the bitcoin market, coupled with either a firesale to comply with the ban or US gov seizure of a bunch of the coins, which will push the price lower for anyone who hasn't sold yet since their buyer pool is now much lower.