I think you'll be fighting a losing battle because that surface is one that cats find appealing as it's easy to scratch at.

I don't know of any effective deterrent/repellent other than sticking canes upright into the ground or using a fine chicken wire type mesh on the ground.

You need larger gravel, even pebbles - cats like pea shingle, its like cat litter, but if you use small pebbles, they'll leave it alone.

I dont have trouble with cats using my peashingle area in the back garden, probably because i have a dog, but when i started new beds in my front garden i had a cat doing her buziness in the same spot (yuk) i bought a product called Keep Off, it was gel chunks which smelt of lemons, i had no cats mess after that.. you can buy it from the £1 shop... Worth a try...

Some of my household solutions, formulas books at home recommend mixing lemon peels and used coffee grounds in a food processor and working it in around the borders. Cats aren't supposed to like the smell.