You can't stop existing Bitcoin users from trading amongst themselves, but it can be made very impractical to acquire Bitcoin with money.
The government can easily isolate Bitcoin exchanges (and any other entity it chooses) from the USD banking infrastructure. Put them on the OFAC list (or similar) and they won't be able to get bank accounts, accept wire/ACH transfers, work with credit cards, or use any other money transmitting service.
Transfers of paper cash in person would be the only option, and a campaign of well-publicized Bitcoin-for-cash sting operations could easily make this too risky for most people.
ASICs are, well, application-specific. It's not exactly easy to fabricate hardware in secret, and there are no excuses to hide behind when you've got a warehouse full of Bitcoin mining rigs.
The government can easily isolate Bitcoin exchanges (and any other entity it chooses) from the USD banking infrastructure. Put them on the OFAC list (or similar) and they won't be able to get bank accounts, accept wire/ACH transfers, work with credit cards, or use any other money transmitting service.
Transfers of paper cash in person would be the only option, and a campaign of well-publicized Bitcoin-for-cash sting operations could easily make this too risky for most people.
ASICs are, well, application-specific. It's not exactly easy to fabricate hardware in secret, and there are no excuses to hide behind when you've got a warehouse full of Bitcoin mining rigs.