The antonyms for "start a fire" can range from the harmless to the extreme. At one end of the scale, we have "extinguish," "quench," "dampen," or "snuff out," which all imply a gentle or moderate action that puts an end to the flames. On the other end, we have more forceful antonyms such as "suppress," "smother," "stifle," or "squelch," which suggest a more vigorous effort to quell the fire, either by cutting off its oxygen supply, flooding it, or by using chemicals or foam. Of course, there may be situations where starting a fire is necessary, such as for warmth, cooking, or signaling for help, in which case the antonyms might not be desirable.