Cats are protected by law, so if you want keep them out of your garden, you must use safe, humane methods.
Before you use any deterrent to keep cats out of your garden, remember that cats are protected under the Animal Welfare Act 2006. Injuring or harming a cat using any home-made or shop-bought deterrent could be considered an offence under the act, so bear this in mind when choosing deterrents.
Here are a few humane ways to keep cats out of your garden:
See also - Garden and outdoors