Could it be made so that you could turn off certain mobs (without going into the code) (example turn off all mobs but zombies)
http://www.cataclysmdda.com/smf/index.php?topic=151.msg1215
Try using keywords for your question in the search bar, or failing that just have a look at least a few pages back in the appropriate area.
I hope that has everything that you need to know.
The mod linked by warwolf0315 does remove certain monsters, but it also removes other things like labs and CBMs.