How to stop a female cat from spraying?

Are you sure she has been spayed? If you are how old was she when the surgery was done? The older the cat is when spayed they will continue to spray afterwards. I had the same problem with one of my cats, she was almost a year old when I was able to get her spayed. She kept spraying for a while after. I got an empty spray bottle and filled it with water. Whenever I caught her spraying I'd spray her with the water and then clean up her mess right away and sprayed the area with bitter apple to keep her away from that area.