The code #409 when dialed from the extension the message was sent to (332 in your case) will cancel the message sent feature.

I am told you cannot disable the ability to set a Msg Light on an extension. You can only disable the extention from sending a Msg light.

I think there is a code to turn off the sent message from the sending extension, but I only know the code to turn it off from the receiving extension.

I would imagine you could hook an analog phone to the port on extension 332 and disable it with #409. This is how I disable it on modem and faxe lines. The MSG light will make the analog phone card "tick", and a butt set or jimmy rigged analog phone will allow you to clear it off of your 66 or 110 block.