So ring your garden with something that makes it uncomfortable for kitty to be there. Cover with chicken wire, plant ground cover to hide the chicken wire from your sight or plant ground cover that is prickly. To save the birds that visit your garden, plant prickly plants at the base of your trees and train them to grow up the tree trunk even if it isn't very big. Roses come to mind :0)Take stock and find out where the kittys like to visit mort. Perhaps they only visit certain parts of your garden,and you will only have to do part of your garden in wire. Give some of your produce to neighbour and tell them they are fertilized with their kitty poop,so you are returning the favour,maybe they will get the message.