Sprays act as a deterrent and work best when they appear to be an 'Act of God', so the dog links stepping onto the stairs with an unpleasant blast of air and not with your presence. Your best bet is to get a can of compressed air that is motion-activated, and site it on the lower step. That way when the dog goes to go up the stairs they get an unpleasant surprise. This will make the dog think the stairs are a no-go area, which is what you want to achieve.