If you are insistent on keeping your toilet paper on a roll hanging near the toilet instead of a sealed container near the toilet, make yourself a toilet-paper cover.
Get a pair of hinges from a hardware store, a small plastic bin big enough to cover your toilet paper holder, and your tools.
Screw one end of the hinges to the wall above your toilet paper holder, and the other to the plastic bin. I suggest using nuts and bolts and a hole punch for the plastic bin end of the hinge.
What you should end up with would be a plastic bin that covers your toilet paper holder and swings up when you want to get at the paper.
Some cats should figure this out, but constantly having the cover swing back over their paws should discourage them.
Another possible design would be the same plastic bin, with one side cut away, screwed to the wall and covering the toilet paper on the sides and bottom so that one can reach into the top, where the cut-out side is. Of course the cat can technically still get at this, but the only way is to stand on the (very thin) edges of the plastic bin, or to stand on the (unstable) roll itself. Should discourage Kitty, but looks weirder than a cover. This is dependent on whether or not the cat can reach the roll while standing on the toilet, and hook the paper out.