I also have 2 cats which use an outside litter tray which is under a table on which is my cold frame but they do occasionally err to the garden. My proven prevention methods are placing plastic netting (plastic clematis netting is good) over the vulnerable plants. Wire hanging basket frames and plastic bell cloches work well on emerging shoots such as peonies, lilies and delphiniums. This year I also placed branches from my Christmas tree and some holly in the open spaces between plants (which I have now removed) and had no problems at all. What also works are small canes stuck in the ground between plants, restricting the space for them to scratch the earth up. My cats cannot get out of the garden, nor any others get in, because I have a product called 'purrfect fence, which enable them to have the rune of the garden and the climbing steps I have put in without the risk of being run over.