It's been a while, so try not to mind my lack in detail whilst explaining this. >.> I think it's in the weather menu, there are boxes for Morning, Day, Evening, and Night. Or 4 names meaning those things. Next to them you pick the lighting for that time of day and the sky color. Simply make them all the same as the 'Night' one. Also, perhaps redirecting the texture for the sun to that of the moon. The moon will just rise and set over and over, but people shouldn't notice because it will be slow. Then the lighting and sky should look the same all night long and you have a worldspace thats perminantly stuck looking like night! Again sorry for the lack in detail, I don't have my geck open and it's been like a year since I did this kind of thing.

Hopefully this can get you started in the right direction.
Gunmaster95