Your cat can scratch doors and windows for many reasons. He might scratch to condition his nails, to mark his territory, to get a good shoulder or back stretch, to express his frustrations or to get to a teasing bird or squirrel on the other side. Regardless of the reason, your pet companion can do significant damage with his nails. There are various things you can do to stop him in his tracks, but it might take some experimenting to find out what works.
Startle Your Cat
Startling your cat each time he tries to scratch a door or window can eventually make him stop the behavior. When you catch your cat in the act, clap your hands loudly, blow a whistle, shake a can of coins or say "no" with a firm, loud voice. Alternatively, hide in an area out of your cat's sight, and when he starts satisfying his urge to scratch, squirt him with a water-filled spray bottle. With consistency, he'll stop scratching the door or window to avoid the unpleasant consequence.
An unpleasant scratching surface or standing surface might make your cat stop scratching the door or window. Stick transparent, double-sided sticky tape on the areas that your cat scratches. When he starts scratching, the tape will stick to his paws, which might make him think twice about continuing. Additionally, place an upside-down carpet runner in front of the door or on the windowsill. When he stands on the runner, the nubby surface feels unpleasant on his paws, making him want to stay off it.
An unpleasant scent can keep your cat away from areas that you don't want him in. Use a commercial smell aversive or a citrus-scented cleaner on the door or window to discourage your cat from going near them. Alternatively, soak rags or cotton balls with cologne and place them on a piece of plastic in front of the door or window. When your cat is up to his old tricks, the unpleasant scent will make him stay away from the off-limits area.
To fully correct your cat's scratching, entice him with an item that he's allowed to scratch. Place a scratching post near the door or window, or hang a scratching pad from the doorknob. Rub catnip on the scratching post or dangle cat toys in front of it to motivate your finicky feline to pay attention to it. When he starts scratching the post, praise him lavishly and give him a treat to motivate him to keep up the good work. Also, trim your cat's nails weekly, so that if he does revisit the door or window, the damage will be minimal.