cats are creatures of habit, and generally react badly to majior changes of any kind. to start with, move kittys litterbox to the bathroom if its not already there. leave next to the toilet for at least two weeks, if the litterbox was in a completely different area of the house, expect an accident or two if your cat is older. say no firmly, and place them gently in the litter box. some cats are particularly stubborn, and might need to be shut in the bathroom with food and water for a night.