Working around light spill from the floor.

[CW]

When using a Chromakey technique, obviating light spill from a rear vertical plane doesn't seem difficult to accomplish. Just move the subject from the vertical plane—perhaps as much as 20' in front of it. However, obviating light spill from the horizontal plane doesn't seem as straight forward as the subject can't be moved above it. Given that I meter light on both planes so that the color is as uniform as practical throughout the scene, how would any reflection from the horizontal plane be avoided? If I reduce the EV of the horizontal plane to a level that does obviate any reflection, which may require several EVs, won't pulling the key be impaired?

Thanks in advance.
CW