I know this topic waaaaay too well. I can tell you that you CAN solve the problem, but you need to understand where they are coming from. When I first moved here I would get flies that were the size of small birds (for TWO years!), I mean these things were HUGE. I tried all the tricks, fly paper, spray, wine, etc. It is all a huge waste of time. In the end you have understand the basics about flies. They breed from decay. This means that something has to be rotting or dead for them to breed. First I checked under my deck. Nothing. Then I checked my chimney. BINGO. There wasn't a chimney cap, so critters would go and fall in the chimney and die, and the flies would breed, can come in the house, probably via the hot water heater connection or something. I cleaned out the bottom of the chimney, and installed a nice stainless steel chimney cap from Home Depot. Never had a single fly again.

Trust me. You have to find the breeding ground and get them there, you WILL NOT kill them for good in the house.

Good Luck,

Pete