I had the same problem - my neighbour's cat kept coming in my garden and digging things up and also using it as its personal toilet... So I tried a cat scarer, like boxtops suggested. I found it worked really well! The cat really didn't like the noise and now goes to the garden on the other side of my neighbour's house to do its business instead! :D
So I'd recommend them. I got mine at this site if you're not sure where to look:
http://www.primrose-l...deterrents-c-102.html
Though I think they do other kinds of cat deterrents too so might be worth a browse.
Good luck! :) Media URL: http://www.primrose-london.co.uk/cat-deterrents-c-102.html
Description: