ShutUp was written on the request of some hardcore raiders who were getting annoyed at people whispering them all the time.
It automatically blocks all whispers from people who are not in your guild or friends list, and sends them an automated reply.
There's no way for people to whisper you any code word or anything to get their whisper through.
/shutup can be used to configure the reply sent, by default it is "ShutUp: Your message has been blocked, since you are not in my friends list or my guild."
If you execute /shutup with no following text, the reply will be disabled. You will still block them, you just won't let them know. If you want to disable blocking, you have to disable the addon completely.