The dungeon delete screen allows you to delete rooms that you no longer wish to have in your dungeon.
If you select a room you can delete a room if you select the entrance room you can delete the entire dungeon.
By deleting rooms it is possible to delete your only means of accessing a room and making a room totally inaccessible, if you do this you can always renovate the dungeon and add more rooms to make a bridge back to the inaccessible room.
You will have to exit and re-enter the dungeon for the room deletion to show up on the mini-map.