Hmm, I see what you mean. It's not really shadows you are chasing, its occlusion. If the elements are all set to cast and receive shadows, and the light is set to cast 100% shadows, the light should not pass through the floors, at least with the Scanline renderer (not sure about ART, it has a few niggles). Make sure the object normals are correctly set as well, that might cause problems if they are reversed.
You will have to set the shadows to 100% for full occlusion. You can adjust the "blackness" of shadows by adjusting the Ambient setting of the material used to texture the ceilings, and any other material for that matter. If it is set to a value of 0.30, that should be the same as a 70% shadow falling on material with an ambient setting of 0.0 (I think).
The only other thing I can suggest is to limit the range if the lights themselves, so they only reach the areas they are supposed to illuminate. A bit of experimentation might yield results.
