Shut down and refuse transactions.
It would require human consensus/interaction to figure out what went wrong - typically by adding a block checkpoint - and start the network back up. Those in a position to patch the checkpoint into their client would do so. Those who cannot would simply force connect to a node advertised by it's owner as "connect to me" and whose position they agree with, which would effectively censor out the unwanted chain from their view.
So centralized human control of a distributed network? If security is the ultimate goal then just make it a central authority which validated and prevents double spends and eliminate the massive cost and overhead of the blockchain and distributed work.
A single low end server could handle all the Bitcoin transaction processing.
I think there are ways to provide 51% protection but human control isn't something I would ever support.
Human control is fallible. Humans can be bribed, beaten, extorted, killed, etc.
The block chain protocol could be improved to detect and invalidate a 51% attack (hint: a 51% attack involves a re-org AND doubling of hashpower). Yeah there may be some side effects but they would be part of the network outside of human control/manipulation.
Yeah just like BTC can't be 51% or economic manipulated with just a couple of millions.
Funny to think that all the BTC value is worth $50 million when just $5 million or even less is needed to kill all this network. SolidScam is no good either.
Until issues like the potential for a 51%, price stability, early adopters having 100 000 of coins for $100 this whole system is a joke.
Arguably, the biggest joke compared to crap like CoiledCoin but still a joke.
If we want BTC to be valid we need to find a technological solution for the 51% problem. No more of this "it will never happen" or "the attacker would gain more by contributing" and other such excuses. BTC is just as vulnerable as CoiledCoin if the miners go away. Don't leave it to chance. Fix it while you can.