You could take pictures of their cats doing damage in your yard, to your fence, using the soil as their litterbox, send the pics to them along with the bill to repair/replace. That is if the other ideas given don't work.
As for kitties getting into your gardens and soil - use aslan's method of keeping them away. Pee in a cup and spread it around the gardens.
Unfortunately until cats are put on the same level as dogs I dont see things changing. We all know cats are thought to be able to fend for themselves and are allowed to roam free. We also know that things are not going to change any time soon. Dogs will always be a level above cats. :sad:
Assumptions do nothing but make an ass out of u and me.
We can stick our heads in the sand for only so long before it starts choking us. Face it folks. The pet population is bad ALL OVER THE WORLD!