Make sure you have no bare earth for them to use. If you do have an area which isn't planted then cover it in something the cat(s) wont like digging in eg. gravel.

We have loads of cats in our garden and they dig everything up. If it's the mess you despise most, I suggest planting things that deter them, in amongst vulnerable areas like flower beds and veg patches. This is from the RSPB: "There is a new garden plant, Coleus canina, on the market. It is marketed under the names Pee-off and Scaredy-cat. This plant has a pungent odour that is said to repel cats and other mammals from the garden. It should be available from a number of garden suppliers." The whole article might help you, at http://www.rspb.org.uk/advice/gardening/unwantedvisitors/cats/catdeterrents.aspx.

If they've been digging up seedling and small tender plants like they have in my garden, the best thing to do is grow them in pots and trays for longer to make them stronger and harder for them to dig up.

With male cats, badgers and foxes it's sometimes effective to warn them they're on someone's turf, by using a man's pee. It worked in our garden, though it's a little gross!

We also have loads of cats in our garden too, i hate them I'm afraid, when planting I always poke sticks around the soil so there is no room for them to do their thing, the trick is not to leave bare soil exposed, hard sometimes but you need to foil them somehow. If you have dug over a patch for planting later on, cover it over if poss. pea netting is good too.

Don't know if this may be of help,but a professional gardener and nursery man ,who broadcasts on our local radio station every week,is trialing a new product for this.It is alleged to be effective on cats,deer,wood pigeons etc.It is totally organic,and not harmful to animals.He was going to put it on an area,that his own cat has started using,and is going to let us know how effective it is,in the next couple of weeks..Unfortunately,I didn't catch the name,but if you are interested,I will make a point of listening more intently,when he discusses its merits,..good or otherwise...Luckily,I don't seem to have these problems,but I sympathise with you...

The shop that used to be called wyevale has got a good deterrent its like a miniature megaphone , it cost me £10 , but it has a silent call when you press the button (in which the cats can only hear ) and a loudish sound , we have tried this out on cats that keep coming into our garden within the last couple of days and it worked.But the megaphone will need a battery.
This might seem bad to use on cats , but it is called a cat deterrant .I am a animal lover but if it stops cats doing their dirties in my garden i will use it.It is NOT harmful to the cats , it just scares the cats away.
I have tried other cat deterrants and they have cost me a fortune and NOT worked .Since we have used the megaphine the cats have not come back