I've tested it.
Placing a planet over the sun doesn't work, the sun is visible through the planet ( and even 9 planets ).
Painting the sun bitmap in black gives a REALLY sexy effect, reduces the amount of lighting, but the flare is still visible. Hm, have to paint the flare black too.
@terran_emperor: will check that out, sounds interesting.

EDIT: The flare is gone, but the lighting is still there, and heading toward the sun, although not visible anymore, makes you go blind.
The effect isn't that intense like before. but still annoying.
Checking the B5 option...
EDIT:
The B5 ambient effect did it, after editing the values a bit.
It's exactly what i'm looking for, the only thing that still bothers a bit is the effect you get if you look into the sun,
although it's way better then before. I assume it's part of the code?
Thx to everyone!