I've noticed that if I create a filled-in shape in one of my rooms, eg. a small cube, and then continue mapping, and then later decide that I want to delete the small cube, that I cannot just right click on the cube and delete it like I can with objects. When I right click on the cube in the 3D view I don't see a Delete option coming up.
I've also noticed that a lot of the tutorial lessons online tell you how to add things but usually not how to delete things.

If someone could fill me in on basic object deletion, I would be very grateful.