While we have two cats, they have NEVER chosen to use our planters as their litter box - but you better believe everyone elses cats just LOVE to poop in the freshly turned dirt right outside my front door. Its disgusting, and I know my visitors just love to have their noses assaulted with the odor of cat feces while standing on my porch (not to mention the flies). I have tried all the "homeopathic" methods: chili powder, black pepper, even something I bought at the petstore that is supposed to deter cats with an odor only they can smell. None of those methods kept the cats away.
Laying chicken wire DOES work, but it looks so ugly - what we ended up doing was placing decorative paving stones where the cats like to go - if there is no room for them to maneuver their bodies to poop, and no room for them to scratch in the dirt, they just find another place to go. Cats love fresh, unpacked dirt - so anything you can put in the pots to keep them out like river stones, gravel, and even chicken wire will work. I love the idea of the sticky tape too, though I would assume you'd have to change the tape out frequently as it gets UN-sticky.
Our cats like to lay IN our large flower pots on top of whatever plants we have planted, killing them and crushing them to death. To keep them out, we will often put one of those "tomato cage" wire dealy's over the top or will "stake" the plants to a bamboo chute to keep them out.