Okay this is bizarre but it works. Apparently if you fill a ziplock bag half full with water, put a few pennies in it, and hang it in a doorway where flies are abundant, it will repel the flies. I guess the logic to it is that the pennies combined with light make the water molecules reflect a zillion different colors of the light prism. Since flies see through hundreds of eyes, it creates an overload for them and they stay away. Try it, it's free.