I have the same problem with my little monsters, and I have found washing the area with biological detergent with a little bi-carb added to the water, then spraying the area with
Petworld's Cat Urine Remover spray works. It discourages them to spray there again. My cats have also been 'snipped', but one of them is persistent in spraying, and I was told he could still have a testicle hidden inside him which the vet may have missed. Apparently, in some cats, this is quite common, so I am taking him to the vet for a thorough check!! Aww poor lil' Whingeee Bobbit!!