As far as I can tell, when you add a wall, what you're really doing is raising the floor up to touch the ceiling. What you'll need to do is zoom out so you can see the entire roomand then click on the "top" of the wall (actually the part of the floor that you raised) and then you should be able to lower the elevations again.
In edit mode, you should be able to zoom the camera out so you can see the whole room. You should be able to click on the top of the wall you made. Then use the window in the upper right to remove the wall.
Thanks! I would never have thought to click the top... kept trying to select from the side.
I had this problem as well. Thanks for the info.
The Dark Blade
"I've felt your mouse on me before, you perv...." - Troy Hickman
Paragon Wiki
Just wanted to see if this is a bug, if there is a solution and warn others of the problem.
If you use the ADD Wall function it doesnt seem to let you ever select the wall again to remove it. I believe I have been able to do this once but 99% of the time it sems once you add a wall it can be permant. Only way I have found to correct it is to delete the entire room.