Maybe install a plugin which disable them, for example this one: http://forums.bukkit.org/threads/mech-customdifficulty-v4-5-0-monsters-too-strong-or-too-weak-for-you-change-it-1240.20135/
This took me 1 min looking trough bukkit.org, there might be a better one or something, dunno.
Edit: http://forums.bukkit.org/threads/admn-mech-mobtoggle-v1-0-simple-monster-animal-on-off-1060.30731/ Better one, disable them totally