I had a cat that would use my bedroom as his toilet. I talked to his vet and he told me that I should put my cat in a good sized room with his litter box and all the things he needs and also put any feces or anything in the litter box as well and he should get used to it.

Cats will continue to go to the bathroom in the area that they aren't supposed to if they smell their scent. I would clean the area really good.