The Lord has many ways that he uses to destroy the wicked, such as earthquakes, floods, war, famine, floods and famine, but he reserves destruction by fire for the most wicked, so that there is "neither root nor branch" left (3 Nephi 25:1).

Here's some other cases where wicked people are destroyed by fire. In each case, there are no righteous remaining at all.

City of Jacobugath

The story is in 3 Nephi 9:9-11.