Koregan

Yes, it is possible to destroy "not self builded" stairs with a sledghammer.
You should start standing in the middle (second tile) of the stairs, then destroy the upper part.
Climb down one part and repeat until the stairs are gone.
 
(If you start destroying the lowest part of the stairs, your character will "jump/teleport" up to the middle part, but you might not destroy it from down the floor.)