For indoor plants I use to cut a hole in a paper or foam plate and put it around the plant. You also might try wrapping plastic wrap over the pot. Some cats don't like getting their claws caught in it. The problem is that cats have been using that dirt and now it has an attractive (to cats) scent. It may take awhile for the scent to go away. I love the idea of planting some catnip faraway from your plants! LOL