What server config do I need to get monsters to not spawn in the main world, but to spawn in the nether?

I have a small server with my wife, and she doesn't like playing when monsters spawn. That's fine, I can deal with Peaceful, and Spawners still spawn mobs that I can farm. But in the Nether, there are no Ghasts or Pigmen. Is there a way to let some mobs spawn but not others?

What plugins and/or configuration do I need to obtain this? I've got a bukkit server with Essentials.

Solution 1:

World Guard will do this for you, for mob Blacklist, just add creepers, skeletons, anything that your wife does not like, and they will not spawn.

Solution 2:

To stop mob spawning in a world, go to the world and using WorldGuard, simply say the following command:

    /rg flag __global__ mob-spawning deny

Be sure to be in the world you do NOT want mobs to spawn in when you do this. To undo, simply say:

   /rg flag __global__ mob-spawning allow