Hi Guest the really short answer is 'you can't totally' that said do a search inside GoY and you will find lots of ideas a suggestions for how to ease the problem. In fact scroll to the bottom of this page. Click on the 'C' then click on 'Cats in the Garden' and all the info is there...

Get a dog.
Or more seriously, try attracting the cats to another area of your garden. If you can buy or get hold of something like valerian root, your local cats will spend so much time getting totally orgasmic in the smells from the valerian root, that they won't have time to ruin the rest of your garden.
I am serious! Not sure where you can buy it but you can certainly grow it.
(Valeriana officinalis, not red valerian which is a totally different plant.)

I was going to say ..get a Jack Russel....but that would just be cruel....to you I mean!