Buy a dog instead?
But seriously, there are some products specifically for this which you can buy that act as a repellent. They smell bad to the kitty and they taste foul and bitter. My sisters cat, Fatty, loves that stuff and is instantly drawn to anything she sprays it on.
When I had her cats here I just blocked up the spaces on each side of the TV cabinet and the spaces beside the computer desk here so that he physically could not get in there.
Failing that, you could always try hitting them or perhaps lightly killing them, slightly?