Plant some lavender in the flower bed, cats hate the stuff.
Depends on the age of the cat I think - next doors cat was very old and deaf ... and didn't seem bothered by them, though I do think it did stop some of the others.You can buy ultrasonic cat scarers that are easier to throw than slabs. Colleagues report that they are very effective.
Supeduper water pistol. Scare the shite out of them and they won't return.