It should be an exhaust fan, and most of them involve a metal or plastic
pipe to vent outdoors, it's usually the last piece of piping involved
with construction, because it can be routed around other pipes and
things, without affecting the flow much.  Often a couple of bends us
sufficient to make it light tight.  If it isn't and you can access the
space above the ceiling, like in the attic then perfect.  Simply cut the
pipe, use an elbow to add a 90 degree bend, then add another so that the
pipe is now headed the same direction, but shifted over, add two more
elbows to shidt it back on course.  Attach the elbows to each other, and
paint the inside flat black.  Attach to the original pipe, since light
travels in a straight line, except through a lens, you will have solved
the problem.  You could also attach this to the outside, where the pipe
exits the wall, or attach something similar to the fan itself.

Another option, get a piece of the filter material used in room
air-conditioners, cut 2 or three pieces the size of the pipe, remove the
fan and put a couple in, seperated by about 1 or 2 inches.  Tie or tape
a piece of string or wire to them first, with a loop in the end, so you
can pull out the fan, and replace them, once in a while, as dust will
clog your light "filter".

W