There are several ways to kill flies instantly, as suggested by the search results:

1. Cayenne Pepper and Water: Fill a spray bottle with water and add two tablespoons of cayenne pepper. Spray the mixture around your house where flies like to appear.

2. Lemons and Cloves: Take two large lemons and slice them in half. Put five to seven cloves in each half of the lemons. Place the fruit where the flies are appearing. Homeowners claim the scent will drive the house flies away within the hour.

3. Vinegar and Dish Soap: Put apple cider vinegar and dish soap in a large glass bowl. Once the flies land on the bowl, the soapy water will weigh down their legs, and they’ll be trapped.

4. Multi-surface cleaner spray bottle: Spray the fly at close range with a multi-surface cleaner spray bottle. The chemicals in the cleaner act as a poison, ending the fly within 6 seconds.

5. Flyswatter: Wait for the fly to land, then approach it silently and slowly with a flyswatter. Swat the fly with the flyswatter.

6. Rubbing alcohol: Use rubbing alcohol instead of water to spray the fly. It kills them dead and evaporates completely.

7. Homemade fly repellent spray: A mixture of dish soap, water, baking soda, and vinegar can be filled into a spray bottle. The mixture should contain a few drops of dish soap and a tablespoon each of vinegar and baking soda per cup of water. A few sprays of this mixture can be an effective fly repellent.

8. Apple cider vinegar fly trap: A few tablespoons of apple cider vinegar, a few drops of dish soap, and a tablespoon of sugar can be mixed in a bowl and placed in an area where flies are commonly seen. Sugar and apple cider vinegar attract gnats; the dish soap traps the flies and causes them to drown.

9. Fruit trap: A rotting fruit may be placed in a jar covered with a perforated plastic wrap. Flies get attracted to the rotten fruit, enter the jar, get stuck inside, and are unable to escape.

10. Professional pest control: If the fly infestation is severe, it may be necessary to call in the professionals.

In summary, there are several ways to kill flies instantly, ranging from natural remedies to chemical sprays.