provided you are not concerned about protecting specific people, an effective countermeasure is to have a large number `n` of people distributed in space, where

provided you are not concerned about protecting specific people, an effective countermeasure is to have a large number `n` of people distributed in space, where `n` is much larger than `d`, where `d` is the number of suicide drones. similar measures can be used to defend against people driving trucks into crowds, or so on.