I have a cat which comes into the garden and rolls itself on the soil where Lily Bulbs have been planted. These bulbs are now showing yet through the soil. I rolls until the soil is smooth. I have tried everything, but it still comes in.
You could try putting some Orange peel or Lemon peel around by were you have planted your bulbs, cats don't usually like the smell of citrus fruit peel.
Had this in my front garden where a cat was using it as toilet. I got some small canes, stuck them in the ground & threaded black cotton between them. The cats can't see the cotton & it seems to frighten them when they touch it.
Carrust's idea is best, make some kind of frame out of sticks or something that stops the cat from actually reaching the soil. You are unlikely to stop it coming in unless you get a dog or an aggressive cat of your own. Water pistols etc will only work at the time, won't stop it coming back unless you're going to sit and wait for it all day.