Never mind zapping.
It's much more satisfying to hunt the annoying flies down one at a time and give them what they deserve.
Step 1: Choose your weapon
The weapon of choice for the seasoned fly hunter is the broad rubber band. Narrow rubber bands can work, but carry much less energy, so are more likely to simply scare the prey off. Like shooting at a tiger with a BB gun, although the fly is less likely to turn round and take your arm off.
When using the bands, the universally-approved method is to hook the band over the tip of the index finger of your favoured hand, and pull it taught with the other, so that the band will fly wherever you point, and you can even sight along your arm and finger.
However, there is still some debate regarding length - longer bands are heavier, but shorter bands travel more quickly.
Pith helmets are optional.