This plugin allow to disable in your mob-arena the items drop of the mobs and the exp drop.
It depends of Worldguard http://dev.bukkit.org/bukkit-plugins/worldguard/
and WG-Custom-Flags http://dev.bukkit.org/bukkit-plugins/worldguard-custom-flags/
The flag are "non-dare-exp" and "non-dare-drop"
The command are
/rg flag non-dare-exp allow|deny /rg flag non-dare-drop allow|deny