You could try pepper - there's stuff in Woodies etc. you can get to put down on the ground.
Also, orange peel is also supposed to keep them away.
I suppose you could try both if you're desperate!
i suppose they don't stink as bad as the cat's droppings...
haa...just a suggestion off the head...as far as i know cat hate all kinds of nasty smell...put some near the fence or corner then they won't come any near the garden