1. Once bitcoin becomes illegal in US, the country's trade partners will likely follow suit.
2. Existing bitcoin money exchanges will shutter, tanking the bitcoin dollar price.
3. Money speculators and money launderers, interested in fiat exchange rather than ideology, lose interest.
4. Bitcoin mining, already centralized & commercialized, will become easy prey for law enforcement. Profitability, already shot due to (1), will not justify operating under the radar. Hash rate plummets.
5. Possible protocol/packet filtering at tier-1 provider level, likely unnecessary.
6. Diehards keep the network alive, but with fiat flowing out instead of in, it becomes irrelevant.
TL;DR: Nothing high-tech needs to happen, no 51% attacks, no crypto-cracking.
None of this is likely to happen, i'm not cashing out
They can't do anything.