Flies, dirty flies

We have had this problem since we moved into our home 13 years ago. Our covered patio faces south. We have flies that cirlce the middle of the patio from late AM till early in the evening from May thru Sept./October. No cat/dog food near by. I have even washed the entire site down with a bleach solution, including lawn funiture, etc. The only temporary solution is to spray a yard spray 10-15 min before setting the table for meals out there. Is there an answer and solution to this icky problem? We live in the Pacific NW. Ice chests full of your fav summer beverage to he/she who can solve this problem.