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.
It's not centralized. It's democratic.
In the case of Luke's attack, there is no doubt which is the attack chain and which is not. The consensus is unanimous. So if this were bitcoin, we would be check pointing the last good block and moving on with life. In case of a threat of a repeat attack, implementation of an algorithm that performs simple spot checks to detect a bogus chain (discussed in other threads, Gavin contributed a few potential criteria), many bogus attack chains could be auto rejected with no human interaction.