Allan C is right about the water. While in the orient many years ago, I noticed that they used a trap which had water, but once the fly got in, it couldn't get out. There are such products nowadays that have a plastic bag you fill with water, & hang it-after it's full, it's disposable. One is the Victor Fly Magnet Bag Trap for about $6. I saw one in a local nursery (plant, not baby!).

If your flies are inside your walls, thus coming into the kitchen, that's another thing. I once lived out in the country in a nice brick house & noticed that what seemed like hundreds of flies kept somehow getting onto the inside of a particular bay window. After a couple of days of swatting with a rolled up newspaper, I finally went outside & looked around. I noticed that there was a hole in one of the bricks near the window. I watched for awhile, & saw a some flies using it. Then I went into the garage, got a can of stuff you use to spray at a wasp nest. With glove on my hand so as to not get poison on it, I shoved the nozzle into the hole & unloaded the contents into the hole. Then, I sealed the hole & caulked around the windows, inside & out. No more flies.