Disallow Natural Spawning? (WorldGuard)

I have made a region of my current town, and set the flag:
/region flag (my town) deny-spawn creeper
But, creepers spawn naturally
However, I can't spawn them using /spawnmob creeper

How can I make NATURAL SPAWNING of creepers (and other monsters, all, etc.) stop?????

Thanks.

Bump?

EDIT by Moderator: merged posts, please use the edit button instead of double posting.