I have been bringing home the used coffee grounds free from Waitrose to deter slugs and it seems to be working for cats as well. And smells nice. And good compost. What more could one ask.
Also perhaps holly cuttings scattered around, and wooden kebab skewers driven into the ground, dull side out, might help. Just don't lean in quickly and poke your eye out.
And finally, I read that large plastic cola or water bottles etc, filled with water and left lying on the ground are deterrents. The cats don't like the slight movement and shininess of the water.