I've dug a hole down to bedrock and have an extra layer of bottom stone slabs on top of the with cobblestone filled bedrock layers but cats keep spawning inside these slabs. You can punch them out and kill them, however they keep spawning and lower the efficiency of the nearby iron golem farm (That's where they come from, at this point in time I have 9 beds set up and 9 villagers claiming these). In the attached picture you can see a cat spawning inside the bottom slab layer (chest in background as indicator).

Note: It says version 1.16.5 because I cant select 1.16.4, which is the version I am actually playing on.