Once you clean an area, how are you going to prevent the cats from going there again?

My cousin just went through something similar; she sent 2 of her 3 cats to live with her daughter for now, until she sells her house and is somewhere else. She knows it is one of those 2 who "misses" the box.

It took her repeat applications, and many different products, to get the smell gone. No one thing worked, she tells me. And what did it on one rug was a waste of time on the next rug but she has no idea why.