What you've found is a [Dungeon.][1] Specifically, a Zombie dungeon. It contains a [Monster Spawner][2] which is able to spawn monsters even if `spawn-monsters` is disabled. The only way to stop a spawner from spawning monsters, is to fully light up the area, or set the server to peaceful.


  [1]: http://www.minecraftwiki.net/wiki/Dungeon
  [2]: http://www.minecraftwiki.net/wiki/Monster_Spawner