stop cats

Since they are your own cats, my experience might work for you. I used a combination of two things cats hate - being "peed on" by a water pistol, spray bottle, garden hose; and they hate a loud sudden noise. I sprayed with a heavy duty water gun from a toy store - because it holds a lot of water and shoots a long way..... accompanied by smacking a rolled up newspaper flatly on the patio table. Eventually all I had to do was the noise, and now all I have to do is ask "do I have to get the newspaper?" Mine hate the loud noise worse than the water. Works best if you can sneak up and surprise them.

Cats don't like a sudden noise and if I want Blodyn to stop doing something I just clap my hands. It usually works. However it may not deter her from diong it again when I'm not there.
Also try planting as many things as you can in the rock garden, then there will be no room for your cats to get at the soil. The rock garden will also look nice : )

Blodyn is right about when you are not there - I found if I left the newspaper and water pistol where they were inclined to pee, they got the message..... just be sure when you fuss you show them the pistol and newspaper so they know where the harassment is coming from.