Or other wisely said: "Security is not absolute. It's just a matter of will and perseverance to break in."
Now to come back to the login banner: what exactly do you want to protect with a "lawful banner" your network/application or the operators ?
I mean, if you offer someone a login and you cannot: - ensure you can clearly identify the person loggin in - ensure that, within the reasonable limit, your authentication system is secure (2 factor authenthication, ...) why bother about statements like "I told you not to" ?
Either you set up your systems in a way you can sleep at night or you will have to deal with a potential security issue. No matter how your banner will look like.
You can actually resume the whole story in two simple words: risk management.
Daniele
Michael Naef wrote:
"Security cannot be declared, one can only try hard and constantly to implement it."