If you have an air pistol, you can fire it at them without any pellets in it. The compressed air itself stuns them.
Might fit better with your dirty harry desires? : )
...as a kid, i had an archer fish - id stun the flies with compressed air from the pistol, chill them in the fridge (at the back, so noone noticed!) and then put them in the above water vegetation on a split layer aquarium - they would 'wake up' and then the archer fish would notice them
stock photos -

