Mr. pware has outlined and excellent method. (pun intended!)
The good news is you can zoom out quite a bit in T:ANE and delete the baseboards.
My method is to create a line of deleted baseboards along edge of what I want to keep first, then carve this up into chunks that I delete. I then save often because a mistake can't be undone while deleting baseboards!
One of the faster ways of deleting, where you can delete three baseboards in a single view, is to get down on the ground and click on the baseboards you can see in front of your camera view. When they can no longer be deleted, move forward and delete some more. By dividing the baseboards up into chunks, and separated from the part you want so you don't click on something in error. Using this method, I've been able to delete lots and lots of baseboards this way quite quickly.
Drawing the line along the border is a good idea though, particularly if you need to step away and come back later.
John