You can buy these little sticks that you stick in the ground. They give off an odour that cats can't stand. Makes them look for somewhere else to go. You might need alot of them though if your lawn is big. Think you can buy them from most garden centres and pet shops.
I've had this problem before, mate. Personally I began a regime of stalking each and every cat to find out where it lives. Then you write a letter (it is important to note that the letter should, upon completion, be soaked in oil to remove any chances of being traced back to you) to all said homes, requesting they lock their cat up or there will be severe consequences. After the letters have been signed, sealed and delivered (again, to remove suspicion, I wouldn't post it near your house), you create a series of Vietnam-like traps and position them around the garden (NOTE: It is important to cover odd places such as walls, and around any upright object - they love upright ****ing objects. If you are wondering where these places are, you're stalking will have helped you find their 'hotspots'). Then, you wait. Eventually when the slimy, curious **** struts across you're garden thinking it's on some sort of catwalk, it notices an upright object - being the nosy, self-centred twat it is, it mosies up alongside it and circles it. When the time is ready and it's just arching it's back, getting ready to lay one on you, you strike; BAM! You release your trap and the cat gets the cream. Take the cat out and take it into your underground operating room (to get full and significant results, it is vital to have one of these). When down there, you basically cut off a paw and send it to the owner in a letter. Hopefully it will traumatise the little girl or whoever opening the letter enough for them to never get a cat again. And we all know, no cats = no cat ****. I'm not much of a cat person though, so you might wanna try something more relaxed first
