Hi William C7G ...and welcome.
This might help also, a good place to get started.
http://www.thegamecreators.com/?m=view_product&id=2001&page=VideoTutorials
But to answer you question, one would pick the segment that
has the floor piece to remove, then at the top of the "EDITOR"
there's a "FLOOR" button to chose. Choosing that one can now
add or remove only floor tiles of the segment your working with.
Do this make sense? If not let us know.
best
You know you're a bad gamer when you're able to run out of infinite lives.