Anything that can free people from tyranny must necessarily also be able to free them from the burden of obeying laws. The reasonable standards of conduct have to actually be built into the system in order for it to have a limit on what amount of external control is allowed.
Systems with smart contracts would be able to specify that a government actor could interfere--even retroactively--with any transactions that explicitly opted in to that jurisdiction. So you could sue someone over their Ethereum scam, and a judge could rule in your favor, and then a clerk forwards the order to the technical magicians for the digital circuit court, so that you get as much of your money back as is technically possible to recover.
If you don't opt in, you are in the land of caveat emptor, and the court tosses your case because it can't help you, even if you deserved it. Or people could opt in to private arbitration. But if someone wants their mathmoney to be outside the reach of government enforcement, it is necessarily also out of the reach of government protection. If you won't follow someone else's rules, you can't expect that your untrusted counterparty will follow them, either.
The first state to create a solid opt-in jurisdiction for network-based civil complaints will clean up, just as Delaware has for corporations.
For the record, the SEC won't ban cryptocurrency. They want technology (in general) to stay here and potentially evolve into something that can be useful and provide jobs and value someday, so you can still free oppressed nations, but you will need to cooperate with the American government if it's being done on our soil.