Don't throw the kitty out yet.
Nature's Miracle is my standby. It has saved more carpets and rugs (and furniture!) than I can describe. The directions may say to spray it or dab it, but what you really need to do is pour it. It needs to soak all the way through the carpet *and* the pad. Apply some to the walls and baseboards, too. (I have a little black light so I know where to aim it.) Usually, once it's all dry, its own smell and the cat smell should be gone.
However, it doesn't save everything. Once in a while I've had to have the professional carpet cleaning fellow come in with his special stuff. Then the closet or room suddenly grows a baby gate or whatever else will keep the kitties out of there. You have to discourage them from returning to the scene of the crime.
Clean your cat box thoroughly once again, and the floor around it, and start over. In fact, add another cat box. It might make them happier. Some people even say you need enough litter boxes to have one per cat plus an extra.
I don't know if the diabetes has anything to do with it (we've had diabetic cats). I absolutely love cats, but sometimes they behave in a manner I can only describe as cussedness.