Not sure about house flies (like the big ones), but with fruit flies I have several years of expertise in killing them.
Poor man's fruit fly trap: Plastic party cup, food-grade plastic baggie, rubber band, apple vinegar, dish soap.
Cut the plastic baggie from corner to corner making a "triangle" and cut the corner to make it into a "cone/funnel". Make with enough overhang so you can place it to cover the plastic party cup and use the rubber band to seal it on. Before this though, pour roughly 3/4" inch deep level of apple vinegar and a drop of dish soap to break the surface tension.
Leave many cups on different areas near the sink, trashcan, etc and watch them collect. EWWW!!