June 21st is St. Leufredus Day. St Leufredus, a monk who lived around A.D. 738, is the patron saint against flies. Legend says that one evening St. Leufredus stopped at a house to pass the night. The saint found the flies so troublesome that he couldn t sleep; but when he bent his head in prayer, all the flies flew away. Try Prayer or try these:

Two ways to keep flies away from your picnic table:
1) Get a small tin with a lid (like a cough drop container), cut a piece of sponge to fit inside and saturate with lavender oil. Cover and let sit for 24 hours, then remove the lid and place on the table. Replenish the oil after each use.

2) Insert 20-30 cloves into a sweet, ripe apple and place on the table. 
In spite of its short life span 22 to 25 days from egg to death the housefly s descendants have survived since the Cenozoic era began 65 million years ago. So if these methods fail, you could always adopt a bunch of frogs. Then you ll really be going green!
Some More Odd Natural Fly Killers
Carnivorous plants, like the Venus fly trap or the pitcher plant, are available to purchase at a number of retailers. This approach to getting rid of house flies is more entertaining than realistic. At least that is my assumption. I could be wrong; maybe they can effectively control a population of flies. Try it and let me know.
Boric acid is a commonly used natural insecticide. Powdered boric acid is placed in places where insects congregate. The powder sticks to the hairs on the insects body. When they preen themselves, they ingest the boric acid and die.