Remove illumination preprocessing on specific space

Hi - I am designing a set of tunnels in complete darkness. Because starting location has some daylight I have decided to include sky into the level, but the rest of map should be dark. Now is the problem - sometimes places which should be shadowed are unexpectly lighted (example below). I decided that instead of trying to patch it up I should just remove main cause of unwanted light, which is daylight itself. So - how can I tell editor that I want to have rendered light on some space, and somewhere I want to have complete darkness?

Here is my tunnel from outside:


(place with hole on right is desired space with light, the rest should be in darkness)

And here is the bug - first picture is with flashlight on, second is after turning flashlight off.
It is expected to be total darkness.

For now I fixed the problem with editing sky mesh to be only half-sphere - i shaped it into much smaller version. But still general question remains, how to exclude some space from light preprocessing.