Isaiah 47:14 Behold they shall be as stubble; the fire shall burn them; they shall not deliver themselves from the power of the flame: there shall not be a coal to warm at, nor fire to sit before it.
Have you ever seen a farmer set a wheat field on fire after it is harvested? The stubble burns fast and then it goes out.
Ezekiel 28:18 says that the devil will be turned to ashes. In verse :19 it says he will be gone forever. Unquenchable fire leaves nothing.