A covering of chicken wire?

Cats are very difficult to train and having cats myself i'd advise using anything that sits 'ontop' of the ground so they can't physically 'get' to the soil.
With my 2 cats i make intricate criss-cross effects with mini bamboo canes, sometimes i lay them on the ground and other times i insert them into the soil so they're slanted/sticking up .... either way, laying something ontop of the area would work.

I used to save my old rose prunings and do the same as Louise,very effective.We only have one cat nearby now,so not a problem anymore..