Take a lemon and cut into two halves. Place them in your oven directly on the rack and leave the oven door open for a few hours (you can even do it overnight!) Take an aluminum tray and place at the bottom of your oven to catch the flies. In the morning, close the oven door and turn the broiler on. Let the oven cool, toss the citrus, and wipe out the base of the oven OR remove the tray. The flies are will be gone - and not to mention, your kitchen will smell heavenly!