any cleaning product that contains immonia for starters. The cats were doing what they term spraying or marking the reason the smell is so strong is because its consintrated urine, the ammonia in some cleaning products smells to a cat as if another has marked its territory so it will continue to spray the area until the only scent is its own. My reccomendation is wet the area with a solution of water and vanilla extract/ essance the type you use in cooking then apply a heavy coating of Bi carb soda do draw out  the moisture then vaccum up. wont get rid of the stain but will get rid of the smell.