Yeah, the first thing I would do is file a complaint against the vet. His advice was completely void of any humanism.
Yes, females spray, even fixed ones, usually in multiple cat households - establishing dominance. Cleaning usually will not stop it as it has little to do with the specific spray area. If it gets too bad, you may need to consider rehousing one of your "children." Or, if you are like us, you just clean it up every time it happens and keep on going.
I chose not to throw either of my real children away every time they made a mess, either. Call me crazy.