You can make a natural cat repellent by mixing essential oils such as orange, lemon, peppermint or lavender with water. Mix 1 part of these oils with 3 parts water in a spray bottle, and spray the areas in which you want to keep the cats away from.
As with humans, there are scents that some cats may hate, which other cats may find appealing. You may need to try different scents and mixtures in order to determine which work best for the cats that you are dealing with. Since cats are not fond of citrus scents, you may consider squeezing a lemon into a liter of water and adding 24 drops of eucalyptus oil. The mixture of citrus and eucalyptus should prove a deterrent to many cats.
Another, somewhat longer procedure involves boiling 2 tablespoons each of cinnamon, lavender and rosemary in a pint of water. Let the mixture cool and sit overnight. The next day, strain the mixture through a clean swathe of cheesecloth. Mix in 1/2 cup of vinegar and 12 drops of a citrus-based essential oil. Once thoroughly mixed, pour the liquid into a spray bottle and spray it onto the areas you wish to keep pet-free.