Face it, a gravel garden by definition *is* a giant litter box. I once discouraged cats from using a gravelled area under my eaves by rolling out chicken-wire on the ground. They didn't like stepping though it.
Originally posted by Rick Beaver: I was considering a BB gun - but I am not sure if it is illegal or not
Use the other suggestions. If you shoot one of them with a BB gun, I will remove your account and find out where you live.
Unfortunately, it is difficult, there are some devices out that that keeps cats or dogs away from a particular area with either sound or scent, you can find them at pet stores. The Dog or goose, or both is a really good idea. Using Bark instead of gravel. putting a fence around your place. Telling your neighbors to keep their cats inside.