Interesting idea. Hadn't considered it. At present, it'll just be a toggleable option. Won't have time to implement something more complete before the next release. I'll bear it in mind for later.
Scratch what I said. It was too good an idea to pass up, so I've implemented it. You can now toggle your Combat State to Fight, Don't Fight, or Selectively Fight, and can select specific monster types to add or remove from your Selectively Fight list at any time. While set to Selectively Fight, you won't swing at any monster types you've specified in the list. The game defaults to Selectively Fight, with any monsters your deity would prefer you not to kill automatically chosen. (It also defaults to adding Golems to this list, since you won't be able to hurt them in combat at start of game, and trying will just wear down your weapon, though you can remove them or any other monster types from the list at any time).