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.