Stick something thorny, like broken up rose stems in the area?
Dull the points to make uncomfortable for the cat to walk over.
They only need to walk in the area once and they'll remember
to avoid it next time. Any other manner of making their feet feel
uncomfortable might work as well.

Another option is to put something there that offends there sense
of smell. Their noses are sensitive. They also have a vorneronasal
organ (Jacobson's organ) in the roof of their mouth where they can
taste what their smelling without coming in contact with the item. I
haven't tried this, but perhaps basil will work. Cayenne pepper or
common black pepper might work as well. Supposedly cats can
even smell common table salt.

http://www.thedailycat.com/mind/comm.../senses_smell/

--
Jim Carlock
Please post replies to newsgroup.

wrote:
Whilst I have no aversion to cats, I do have a problem with them using
the pebbled area in my garden as a litter tray! Can anyone please
advise me of a way to deter this from happening?