Short of a ubiquitous gorgon stare[0] level of surveillance, how do you stop someone from deploying an autonomous drone with facial recognition that hunts down a particular individual?
Don’t drones require registration in the US? So any event like this will become a cyber investigation. And for the first one likely even considered domestic terrorism so it would use the NSA’s gorgon stare to trace back the purchaser and operator.
Below 100 feet it is remarkably difficult to detect and stop a drone in time to defend an attack. Especially since they can be fully automated, have onboard inertial measuring units and computer vision. Your best defense would be hunter killer drones over high value targets or trained birds. But defense always lags offense.
Guided missiles used to be an expensive proposition. Now they are off the shelf parts for a couple thousand on digikey.
https://en.wikipedia.org/wiki/Gorgon_Stare