If you have the code source of the other plugin, open the drop cmd callback, and instead of return PLUGIN_HANDLED, put return PLUGIN_HANDLED_MAIN, it will block the command from being sent to the game, but other plugins will be able to catch it. Do the same in your plugin.
Or if you want that players can drop their weapons, put in both : return PLUGIN_CONTINUE.
My question is out of context of this thread, but relevant to another on the forum. (one i'm still trying to figure out) Will using PLUGIN_HANDLED_MAIN work in the case of showing an X when someone does damage to an opponent? Link to my code.... http://forums.alliedmods.net/showthread.php?t=184082