How to stop cat using my gravel as a litter tray

[StuntBottom]

The bottom half of my garden is gravel and one of the neighbourhood cats has started using it as a litter tray. I have a cat of my own but I don't think it's him as he uses an indoor litter tray. He does go in the garden, though, which limits my options, eg I can't use any of the ultrasonic cat repellent devices. At the moment, I'm clearing it daily and spraying the area with beach and water, hoping that the smell of bleach puts it off. However, we're about to go on a two week holiday and it's going to be disgusting by the time we get back. Anyone got any ideas for something effective but that won't hurt my own cat?

In the past I have used white vinegar a huge container of it and poured it all over the gravel. The whole street pinged a bit but it seemed to work. We have moved since , cat's no longer bother us due to the dogs.

20mm gravel.
They don't like it.

Will try to white vinegar. Have just bought cat repellent granules to try as well. Replacing the gravel is a last resort as it's quite a large area and I can't really afford it.

Spray it with a water pistol (and I say this as a cat owner)