Cat urine streaked onto a wall is usually a sign of a male cat marking his territory, but there could be other reasons for the urination as well. Talk to your veterinarian and try to find out why your cat is urinating on the wall. If you fail to fix the problem, the cat is likely to mark the wall again. Once you have solved the problem, you can clean the wall using homemade or store-bough enzymatic cleansers. If the smell persists, you may need to prime and paint the wall.
If you catch your cat in the act or arrive on the scene shortly after the incident, quickly blot any wet urine off the wall with paper towels or a clean rag. Blot the area rather than rub it so you don't spread the urine around the wall, making the problem larger.
Spray the area with a solution of 1 part hydrogen peroxide to 2 parts water. Spray the wall thoroughly and then wipe away the cleaning solution. If a urine smell remains, repeat the cleaning process with undiluted distilled white vinegar.
If you still smell urine after trying home remedies, visit the pet store and purchase an enzymatic cleanser that will break down the urine. Spray the cleanser on the wall, allow it to sit for the time indicated on the label directions and then wipe the cleanser off the wall.
A urine odor that outlasts an enzymatic cleanser requires drastic action. Try applying a clear sealant to the wall to prevent odors and stains from coming back. If that isn't enough, prime the area with a stain- and odor-blocking primer and repaint the wall.