I can't tell you how to get rid of the smell, (unless bleach or disinfectant will work- the smell is the waste product of bacteria in the urine and this should kill the bacteria) but can advise how to keep cats away. If you have your garden close to your front door, bury moth balls half an inch under the soil nearby. They will be able to smell them and they don't like it and it doesn't hurt them. Which is important. I don't like cats, but wouldn't do them any harm. They are a part of natures plan.