A serious solution is simply for several major stakeholders to publish signed endorsements and/or kills to blocks. Users can subscribe to them, and if a supermajority agrees to kill a block, the merchant at the very least can be configured to stop and go into safe mode.

I have thought of this long ago, now others might take the idea seriously. It was aggressively rejected in the past under the pretense it was too "centralized".  I believe we need it to raise the bar on the risk of a 51% attack.