Are you trying to stop cats from spraying in the house completely? You are fighting a losing battle. You cannot stop them from spraying completely. A more effective approach on how to stop cats from spraying in the house would be to work on tactics to discourage cats in the house from spraying in those areas which you strictly want them to avoid. For example, you would obviously not want them to spray on your bed where you sleep. You would want to sleep peacefully at night. With your bed smelling of cat spray, you would not be able to sleep peacefully. Cat spray smell would be pretty strong and can distract you from your sleep.
Are you thinking about getting your cats sterilized? Beware of the animal society in your city. They are cracking down on cat owners who are using the cruel sterilization method to stop cats from spraying in the house. Even though sterilization solves more than 80% of the spraying problems of your cats, it is considered as a cruel practice in the pet community. Instead of sterilization, you can experiment with the usage of litter boxes to stop cats from spraying in the house. Many cat owners around the world have reported that litter boxes work pretty well in stopping cats from spraying in the house.