The suggestions above - while correct, will affect more things than just the ceiling part - turned off GlobalShadows for instance turns off every shadow, which might not be a wanted side effect.
A solution that only affects the part you care about would be to use a BlockMesh into of a Part:
If you’re worried about things like collisions of the ceiling (since the Part’s actual size would be fairly small), you can put a transparent part where you want to block players in.