Cat urine is so
concentrated that the smell is unmistakable and never seems to go away, now does it? Because of that
concentration, this is why you can never seem to get that smell out! But here are some suggestions...
A wonderful product called natural magic comes in a container which is opened and placed near the smell to absorb the smell. I suggest taking the entire lid off and setting it on the floor near your cat's accident or smirking revenge against you.
There is also a product called Simple Green which when used concentrated will extract the smell from carpet and clothing in the wash. And remember, when washing clothing, you must wash it a few times before you wear it again in order to completely extract the smell.
One final step, if you are replacing or installing carpet. They now have special padding which goes under carpet and will stay resistant to absorbing anything liquid for years. See your local carpet store and they should carry a brand of this padding as well as a cleaner for these things.
Remember to constantly change your cat's litter, and if the cat still returns to those spots, try to place a fresh litter box on top of the spot and use a litter which is specially made to attract cats. Pet stores sell a product called Cat attract, and it definitely works as well as containg a booklet to give you information on how to take proper care of your cats when moving, feeding, cleaning, etc.