> How do I discourage my cats from hunting bats?
Bats tend to hang in places that should be inaccessible to cats, such as attics, chimneys, hollow trees and similar. The best way to discourage your cats from hunting them is to exclude the cats from where they rest. Given the difference in size, that should be straightforward.
Also, make absolutely sure your cats are current on their rabies shots, and up-to-date on their systemics against fleas, mites and other vermin.