Our household has twice as many felines as people. Our two youngest kittens have the tendency to like to dig in the soil of our few remaining potted plants
. After filling a good portion of our vacuum cleaner bag with the good kind of dirt, I decided to find a way to keep them out.
I considered cutting a piece of cardboard, wood, or metal to fit around the plant but watering would become more of a hassle. Then I used a collection of marbles and polished pebbles. They worked well at letting the water pass through and prettied up the top of the potting soil but the kittens loved to rummage them out and play with them. Finally, I threw a few large pine cones in one of the pots. They lock together well enough that the cats don't pull them out, they don't prevent watering, and, as they degrade, they will add organics to the soil.
With the idea of interlocking items in mind, my cacti are now guarded with beach-combed coral pieces and the aloe plant has an arsenal of green army men protecting it.