Bukkit

I am quite confused by all this coding things regarding the config.I'm kind of a noob regarding, so I'd really appreciate some help. I'd just like to know how to make it, so only Zombies will spawn and no other hostile mobs. Non hostile mobs may spawn, just no skeletons, creepers, endermen, spiders, slimes and silverfish can spawn. I'd really appreciate the help. Thanks in advance :D

Paste this into the spawns.txt file

<event>
  <outcome affected_mobs="skeleton, creeper, enderman, spider, slime, magma_cube, silverfish, blaze, cave_spider, giant, pig_zombie, witch, wither">
    <actions>
      <cancel_event/>
    </actions>
  </outcome>
</event>

You can allow a mob by removing its name from the list.