Ask Mr. Smarty Plants

The most attractive garden elements for snails and slugs are good hiding places. These creatures spend most of their time lurking in moist sheltered spots and do their hunting and feeding during nighttime hours. If you remove or otherwise make unsuitable the places snails like to hide, they will simply go elsewhere. Unfortunately, the structure and growth habit of prickly pears often provide good shelter for these slimey little mollusks.

There are several non-toxic (to humans and other vertebrates) control strategies that you might employ to kill or repel your garden snails. Here is a link to an excellent article from The University of California on snail and slug management. One of the copper-based strategies described in the article, along with removal of hiding places, will likely yield the best results for you.

You can cut your cactus to the ground. It is possible that it will resprout and grow assuming that the snails have first moved on elsewhere. In your area (Austin, TX), if it does not resprout from the stump, prickly pear is easily propagated by simply placing a pad into contact with the soil in the desired location. It will soon sprout and begin to grow and before long you will have a new plant and a source for nopalitos.