We have a similar problem with one of our cats, I was told by the vet, to clean the area with biological washing powder and then wipe over with surgical spirit, which will completely remove the smell. Then you could try, sticking bits of dried food to a piece of card and leaving it in that spot for a while - sounds crazy I know but most cays won't pee etc where there is food. Or try spaying Feliway spray on the area, after cleaning it. Feliway is a pheromone spray, which has a calming affect on cats.