This has always been one of my sticking points with the Carrara Realistic sky system.  You can disable the ground, but to get rid of the annoying horizon line, you still have to set the horizon to some very large negative nimber.  It's kind of trial and error.  I normally increase the size of the plane I am working on to extend as far as necessary and drop the horizon to about twice the x or y axis size of the ground plane I am working with.  So If i have created a terrain in inches which is 1000 inches long and wide, I would drop the terrain to -2000 inches and play width the camera height until I had the desired effect.  I wish you could just turn the darn thing off completely, Its not a very good horizon anyways. :)