Try moth balls. I always use them. It deters the cats fouling too. Just dot several around the area you want to protect. They are alot cheaper than all those cat repellents from the garden centres etc.
There is a plant called - coleous canina - which does the opposite of catnip. So if you plant some of these in your garden that should keep the cats away. Look it up on the web to see if it suits your needs.