I dont think you just want to remove the smell
(( you have to remove the source, not just mask the smell. If the smell is that strong that air fresheners wont mask it, it is likely in some fabrics in the house, ie: carpets. If you have carpets, start there, get in a good steam cleaner, I always use Ram but there are many good ones out there.
Then as others said, wash down walls etc. Cat urine is very persistent however, much more so than dog urine, and if the problem persists, you will have to remove carpets later on as it has probably soaked through to the underlay. But start with a good steam cleaning.
Oh, and do be sure to mention to the carpet cleaners what you suspect is the cause of the odour. They will then put something extra in their cleaning product to address that issue.