Don't accepts your cat's behavior - force it to use the litter box
With the secret you can learn about cat not using litter box anymore at the hyperlinked site you can surely become a better pet-owner. You might be wondering, what are these secret, where do I obtain them? The answer to that question is that the secret to getting your cat to use the litter box for life can be found over at Help My Cat. They offer a wide variety of tips, hints and tricks to getting your cat to use its litter box for good!
One of the tips is to place multiple litter boxes around the house. E.g. one litter box at the front door, one at the backdoor and one upstairs. This way your cat will be able to find a litter box to use much quicker and won't accidently end up pooping or peeing on the floor.