Answer:
A couple of reasons why your neutered cat is spraying is because there is still some testicular tissue left over, or he was neutered too late. Cats need to be neutered before they reach sexual maturity at six months old.
One answer: Let your cat outside. It's an awful thing to do, but spraying is worse. Typically, he'll do it outside.
Another way is to clean the sprayed surfaces with washing powder, or a special spray that will dissolve the ammonia (this is undetectable by humans, but helps a cat find his or her way back to the spot). Afterwards, you can try applying a strongly-scented citrus spray or liquid on these surfaces. You can get these from most pet stores.