KeepItems is a lightweight plugin that allows you to force specific user groups to keep their items, even when they attempt to get rid of them.
It is based on a request on the BukkitDev forums.
Scope of what the plugin is capable of affecting (you can turn each one on and off using permissions):
This plugin requires permissions to be set, otherwise it will do nothing by default.
If any of your player groups are given the *.* permission, you must manually negate the permissions shown above in order to prevent those players from being affected. This is how the Bukkit permissions system works, and is not a bug with Bukkit nor the plugin.