J.B.
I'm not sure what you tried from the pet store, but OUT is the best thing on the market. It's probably the enzyme cleaner that was mentioned. I had a cat that chose a particular corner and we shampooed the carpet and tried everything. This was the one thing that worked. You have to be sure to soak the spot with OUT though - as deeply as the urine went - in order to completely saturate it and get all the urine. For just keeping them off the chair, I've used two-sided tape, saran wrap, and other things that cats aren't fond of. Once they realize they don't like that spot anymore, they'll start to stay away.
Also, cats don't like to pee near where they eat. I tried placing a small bowl of cat food near the carpet spot and that seemed to help deter them as well.
One more thing . . . sometimes cats pee in bad places when they have UTIs or kidney disease. (I know you mentioned the flooding, though.) I thought one of our cats was having trouble adjusting to our new house, but found out later she was having kidney failure that caused her to go wherever and whenever she could. :(