Is this for a new plug or one that is currently out? Plug = Plugin Also by "locking" what do you mean? - Making it so others cant destroy - Making it so it doesn't drop "money, coins"
Mobbounty has this feature. http://dev.bukkit.org/server-mods/mobbountyreloaded/ I believe Ecocreature has it as well.
There are also a number of anti-farming plugins (search Google for bukkit anti-farm). Also, OtherDrops can do it - it's a bit trickier to set up for anti-farming but gives you more flexibility.
Sorry for the long delay but I did not find any plugin that works. What I need is that mobspawners not give money (money of the plugin Iconomy). To also avoid imbalance in the economy of my server.
My plugin can do that, if you let mobs drop items like emeralds and buy them at a servershop. We actually use it exactly that way. You can set, what kind of spawnreason is appropriate for modifying the creeps. http://dev.bukkit.org/server-mods/megacreeps/
