"NO" stated calmly in a *low voice", rather than shouting, usually successfully registers disapproval with our two without scaring them. They're smart enough to understand that a serious disapproving low tone means "not allowed". If repeated often enough, they get to learn to not do the thing we disapprove of. So they understand for example, not to walk on my lap top when it's on a desk. To avoid hot cups of coffee placed near a seat when jumping up on the sofa. To stop crying to ask to go out after "lockdown" (they have a kitty tray in the bathroom for night visits).

It's important to offer them an alternative scratching place though.

If he knows not to do it, but he's being naughty, I'd put a plastic or rubber backed mat in front of the door, and a wet tea towel on that. He'll get used to avoiding the damp.