Instructions
Press "D" to open the designation menu.
Press the "Up," "Down," "Right" and "Left" arrow keys to highlight the downward staircase you want removed.
Press "Z" to remove naturally dug stairs. Naturally dug means stairs your dwarf simply channeled, while constructed refers to having used wood or other material to craft the stairs for durability. The "Z" option removes stairs through piling the dirt back onto the ramp.
Press "N" to remove constructed stairs. This option physically removes constructions and will work on anything else you have constructed in the game.