Plugin was requested here:
http://forums.alliedmods.net/showthread.php?t=65740
Any player can say !resetrank in the game chat and the plugin will run the server command: ma_resetrank <STEAMID>.
Tested only on CSS, but will probably work on other mods with a "standard chat" structure (like TF2). If its not working for your mod, you will probably need to tweak the way the plugin extracts the chat message from the say command args.
The rank resets are logged in the server logs so you can see them via a remote log (HLSW) and track when people reset the rank.