Housefly Repellent

We just moved into a new home, and we are having a problem with this strange vortex of houseflies that swarm in the center of the garage, anytime the weather is pleasant. Of course they migrate into the house anytime the door between the garage and the house is opened, and we have two kids. Need I say more?

Any ideas on a natural way (non-chemical)to keep them from appearing in the garage, or even an effective barrier to keep them from migrating into the house? Really, I have to hold my breath when walking through the garage, or I'm afraid of inhaling one. They are that dense. But only in the very center of the garage. Weird. Thanks for any suggestions.