The bad news first: you cannot stop a cat scratching. It is not mischief or revenge for a closed door. That is how a cat sheds the dead outer layer of its claws, stretches its back and shoulders, and leaves a mark — there are glands in the paw pads, and the trace is visible to other cats.
The goal is not for the cat to stop. The goal is for it to do this somewhere you do not mind.
Why the scratching post is not working
Usually one of four reasons.
It is in the wrong place. A cat scratches where it lives: by the sleeping spot, by the doorway, on the route through the room. A post in the far end of the hallway is never used — the sofa in the middle of the living room always is.
It is too short. The cat needs to stretch to full length. If the post is shorter than a stretched cat, it will choose the back of the sofa, which is taller.
It wobbles. The cat puts its whole weight on it. A structure that rocks puts them off the first time, and there is no second.
Wrong surface. Some cats want vertical sisal, others horizontal cardboard, others carpet. Look at what the cat already scratches and match it.
Step by step
Put the post right against the spot the cat is ruining. Not across the room — against it. It should be taller than the stretched cat and it must not move.
Make the ruined spot unpleasant: double-sided tape, a slippery throw, stiff paper. Cats dislike sticky and slippery under the paw.
When the cat uses the post, praise it and give a treat — at that moment, not a minute later. Cats learn well from reinforcement, just more slowly than dogs.
After two or three weeks, once the habit holds, you can move the post — ten to fifteen centimetres a week, no further.
What not to do
Do not push the cat's nose into it and do not hit it: the cat links the punishment with you, not the sofa, and starts scratching while you are out. Do not spray water — it works exactly as long as you are in the room.
Do not substitute claw trimming for a solution. It reduces the damage but not the need.
About declawing
Declawing is amputation of the last bone of every toe, not a manicure. It changes how the cat places its foot and often leads to chronic pain and litter box problems: digging hurts. It is banned across much of Europe. If it is offered to you as a solution to a furniture problem, find another clinic.
When it is not about the furniture
A cat that suddenly starts scratching everything, marking, stops using the litter box or hides — that is no longer a scratching post question. An abrupt change of behaviour can come from stress or from pain. Show the cat to a vet before rearranging the furniture.