Try this to prevent flies from coming into your house. Fill a quart plastic freezer bag about 1/2 full of water and close bag. Hang this bag on a nail near your back door (the one you use the most but would not hang near front door). Flies won't come into your house!
Several years ago, I attended the "Great American House Party" in Philadelphia, MS with my daughter and a friend. It is a compound of cottages/homes that are used a few weeks a year - mostly around 4th of July. These residences have a history and are family owned - handed down from generation to generation. Camping dwellings, in other words. The owners open their homes to everyone who comes along and offers them refreshments, conversation, friendship - Mississippi style. It was a wonderful experience.
To make a long story short, I kept seeing these bags of water hanging beside the doors and asked why they were there. They told me to keep out the flies! Sure enough, no flies were inside!
You might want to try it to see if it works for you!