The light you're seeing is one that gets added to rooms that don't have any lights. The only way to get rid of it is to add a light to that room. I'd suggest going into Plan View and using the Add Light tool and then turning that light off. The Added Light won't show up in your 3D View (unlike an Electrical Light), and you'll be able to get a pitch black room in PBR.
I'm not really sure what could be causing the fog you're seeing. Have you tried messing with Camera settings like Bloom, or turning on/off Generated Sky?