I've found Pepper Dust works a treat after only 2-3 days, but needs to be reapplied if it rains. Cheap enough though.
Like others have mentioned, I also place short lengths of cane/sticks in between young plants so they can't get into a clear patch easily.
My parents have used the ultrasonic system and say it works brilliantly. Doesn't harm any wildlife, nor the cats, but they hate the sound. Problem is, some people with sensitive hearing can (apparently) hear the noise too!
When I see a cat trying to make its toilet in my beds, I pull out the hose and aim it at them - they are normally out of the garden before I've even turned the water on 