If the issue is outside cats, keep the window coverings closed and remove your shoes before coming into your house so that the cat doesn't smell the other cat. You can also try various cat deterrents to keep the cats away from your windows and doors.
Often, felines will spray outside versus inside your home if given a chance. If you can find a way to let your cat outside safely such as using outdoor cat enclosures, your cat may do his or her spraying in your yard instead of your house.
If your cat is spraying in a particular spot in your home, you can try using cat deterrents You might also try putting some aluminum foil over the spot or even move the cat's food dish to the area since cats don't like to have their food near their litter boxes. However, be forewarned that your cat may just start spraying in a new spot.
If your cat is suffering from anxiety, you may want to try Feliway which contains pheromones that calm cats. Also, Bach's Rescue Remedy has been shown to calm anxiety in some pets.
Another solution is to use diapers such as Simple Solutions Disposable Diapers. Although this option can get expensive, the diapers may not cost as much as replacing furniture and flooring.
Spraying is a natural cat behavior that doesn't blend well with indoor life. With the many options available to stop the behavior, you will hopefully be able to find a solution that will save your possessions and allow the cat to remain in your home.