Jeremy, I would evaluate the source of the insects and start there, if possible. If you are certain that they are houseflies, look for the breeding sources....garbage, animal waste (farm and pet), compost piles, etc.

We all know where mosquitoes come from. If the removal of even very small puddles of standing, and treating larger bodies of water with biological larvicides are not possible, then throwing toxic chemicals at the problem won't work.

I'd offer your guests personal protection in the form of botanical repellents that can be used safely. Fans can help, too.