I have two cats, Sabre and Jake, brothers and Siamese-Maine Coon mix. Jake has no problem using the litter box, but Sabre will sometimes refuse to, even if it's completely clean. He'll protest by urinating on the wall, which results in my mother locking him in the bathroom with the litter box until he uses it. We've tried cleaning the litter more frequently and even changing it, but to no avail. This is beginning to happen more often, and my mother is considering giving him away if he continues this behavior. Is there anything we can do to stop this behavior?