This article is from the Problem Behaviors in Cats FAQ, posted to rec.pets.cats newsgroup. Maintained by Cindy Tittle Moore with numerous contributions by others.
Cats are frequently fascinated with houseplants. However, you may not appreciate the attention, which can range from chewing on the plant leaves to digging or peeing in the plant soil.
For chewing, try spraying Bitter Apple or Bitter Orange (there are formulations specifically for use on plants) on the leaves. Dusting cayenne pepper on them may also help. You may wish to grow some grass or catnip for them as an alternative; plants do provide them with needed nutrients (the absence of which may be causing their grazing).
For digging or urinating, cover the dirt with aluminum foil or gravel. If the plant needs it, add some vinegar to the soil to counteract the ammonia in the urine.