Many stray cats, especially males, spray urine to mark their territory. Since you probably do not want to spend the money to have a stray cat neutered, you will have to make changes to the environment that the stray cat is spraying. There are several methods you can still use to decrease the stray cat's inclination to spray the area.
Fight back with your own unfriendly scents. Prevent the cat from going near the area that you do not want to get sprayed. Using strong cleaners or citrus-scented air fresheners will offend the cat's sense of smell and possibly cause it to avoid the area altogether. Setting a device such as a sprinkler will also keep the cat away, especially if you do not want it to spray your yard.
If the stray cat is approachable, make the cat smell the urine by placing its nose near the area where it has sprayed. Use a sharp tone or voice to say something that will send a negative message to the cat, such as, "No!" or, "Bad cat!" You may also lightly tap the cat on the nose, but do not strike its back or any other part of its body. If the stray cat is not safe to approach, fill a spray bottle with water and squirt the cat when you see it spraying.
Deter threats from other animals. If there are other animals in the area, especially male cats that have not been neutered, the stray will feel more inclined to claim its territory. Try to keep dogs, raccoons and unfixed male cats away from the stray. Do not leave out food that may attract these animals.
Call a local shelter or animal service. If the spraying problem becomes so extreme that you want to have the cat removed from the area, there are services that will pick up stray cats. They will usually try to find them a home, but may have the cat euthanized if no one adopts it.
Consider neutering or adoption. The best way to stop a cat from spraying is to have it neutered. If you do not want to chase the cat away or repel it, then you may consider paying for the surgery and then keeping the cat. If you do not want to take permanent responsibility for the cat, you should at least monitor it during its recovery. Once you have put the effort and money into having it neutered, though, you may want to keep the stray if you think you can domesticate it, since most male cats become less aggressive after the neutering process.