Cat urine has a very lasting effect. I occasionally get a tire on my truck "marked" and it smells for ever.
Unless you rip up the flooring, which is acting as a sponge, the only other thing is masking agents. Stick a bowl of baking soda in the closet, it works good for absorbing odors, but will not rid the odor. Rub the area with concentrated Pine Sol,
Sorry, send the cat to the lady that is over run with mice.