Ryskie,
Are you referring to your message notification button as your mailbox?
So if I understand your question correctly you want to stop people from leaving you a message to call them back? Unfortunately, as long as they have access to the feature then they will be able to message your phone. This feature comes standard with all "Executive" style phones; e.x. 5340, 5360. Any other style phones would need the user to enter the feature code either through a button programmed for it or they would have to enter it manually.
There is nothing in the system that would allow a programmer to stop your phone phone from being messaged.
Now, what you could do is press the message notification button without going off-hook and it will tell you where the message is coming from. So if your display shows 5 messages and you press the message notification button then it would show 4 from your mailbox number and when you press it again it would show 1 from Jane down the hall. If you hit it again it should go back to the voicemail message and then you just hit the # key to lite to your messages. Once you hang up from that you can either return Jane's call or just clear the message out with the feature code that you have been using.
Thanks,
TE