The antonyms of "sets on fire" can be stated in various ways such as extinguish, put out, smother, quench, douse, or suffocate. These words depict the opposite action of igniting or burning something. When firefighters rush to a scene, they work hard to smother the flames with water in order to extinguish the fire. Similarly, people use sand or a fire blanket to quench a small fire. To put out a campfire, you need to pour water on it until the flames are completely doused. Thus, the antonyms of "sets on fire" imply stopping or preventing a fire.