There is a simple soultion to this problem:

Buy a cheap second-hand radio controlled car from Ebay. Next buy a cheap second-hand stuffed cuddly toy dog from Ebay. Stick dog onto radio controlled car by some means (just make a hole in dog and stuff car up him...for example). Put radio controlled dog in garden. When offending cat enters garden, chase away with radio controlled dog. After a while you'll hope offending cat visits more often cos it can be fun and quite addictive.

p.s. You can substitute dog with any other animal: lion, horse, elephant...giant mouse?