There are a variety of scents that are unpalatable to most cats, from the unwanted smell of a dirty litter box to natural smells such as citrus, eucalyptus, peppermint, lavender, banana, lemon thyme and certain cleaning products. Cats have an incredibly acute sense of smell, with some reports indicating that they smell up to 14 times more effectively than humans, so the presence of an unwanted smell may have a strong impact on a cat. Some of these smells can be deployed in the process of cat training, specifically as a means of repelling cats from certain areas where its presence is not wanted.
One way of using smells to deter a cat from going in a specific area is to soak cotton balls in an essential oil in a scent, such as eucalyptus or lemon, that is known to repel cats. These balls can then be placed strategically to deter the cat from going into that area. For example, making a couch smell like an undesirable scent may train a cat to stay away, making issues with scratching less of a problem. Some undesirable smells, however, can have an adverse effect on humans as well. For example, a cat repelled by its dirty litter box may decide to relieve itself in a less smelly part of the house.