bottles full of water...i think if u leave them aying about..or buy one of them cat scarers..which are supposed to be so that only cats can hear the high pitched noise..
Hot Chilli powder scattered on dry earth, but if it rains, you need to replenish.
Or I have heard that several coloured rubber snakes (kids toys) placed around frighten them.
Don't put down hot chilli powder - its very cruel. As long as cats are in your garden you can't stop them pooing. The best thing to do is buy a kids water pistol and spray them when you see them, to discourage them from entering your garden in the first place.
Buy a vicious Pitbull terrier from a guy with tattoos who breeds Pitbulls for illegal dog fighting. Put the Pitbull in your garden and see the results after 1 week. Guaranteed to work! Other dogs can do the job but Pitbulls are the best.
You can get pellets (they look like slug pellets) which are impregnated with certain essential oils which cats find distasteful. Sprinkle them around your borders and anywhere where they tend to "go" and it will mask their scent marks and deter them. They are biodegradable, harmless and inexpensive - try B and Q, Homebase and other garden centres.
There are electronic devices available that operate by emitting a high picthed sound which humans can not hear. There is a PIR switch on the device so when any animal enters your garden it trickers the sounder that scares off the offender so it only works when there is a cat or dog there, saves on the batteries!!. Try your local B&Q or garden centre or maybe an internet search engine like Yahoo or Lycos. Hope this helps.
Actually, the single best way to discourage them is to cover the garden in your own urine. The animals will recognise the urine smell as being a territorial marker and will leave it alone. You don't have to repeat this too often either - once ever few months - but first time, you should do it every couple of days for ten days or so. Honestly, it works!
Actually, the best way to keep cats out full stop (aside from persistent watching and throwing a bucket of water over them) is simply either sending the dog out when you see one, or putting dog pooh around the perimeters of the garden, where they come in....deters the ones in my garden from bugging the birds in our aviary!