How to permanently get rid of light fixture?
The cable in the wall have to be physically removed all the way to the other end, or if that is impracticable, as much as possible of both ends of the cables must be removed.  It must be beyond any possibility of anyone ever energizing those wires again.  

If there are multiple cables in the wall, then I mean both ends of each cable, including the far ends that are in other boxes.  

If there are 2 cables and they need to remain in service, you can evaluate whether you have enough intact cable sheath to install a Tyco in-wall Romex splice.  Those can be covered up.  

Otherwise that through-cable run needs to be entirely replaced end-to-end.