I had a bark and gravel area that had become the communal cat comfort stop,

I just got a couple of rolls of the sturdy nylon plastic mesh fencing with about 2 cm holes in it, about 1.5 metres high and 3 metre long roles. The sort of stuff that people usually put vertically between poles as a temporary fence. I laid it horizontally across the flower beds and bark in question, with rocks under it at points to keep it a couple of 6-10 cm above ground level. Before placing it, I removed all the cat poo and hosed it down to try and eliminate the smell.

Cats *hate* trying to walk across that stuff. Left it there for about a month until they had been well and truly broken of the habit of using that spot.

Worked well.

I believe you can also get small electric fences, that run off D-Cells, specifically designed to be erected to keep cats and rabbits out of flowerbeds and veggie patches. They might also be worth a try.