I agree with both the above posts - I would either change the logon page to tell people not to log in and/or stop the normal APP service and replace with a static holding page (and start up a new APP service on a new port that only your "special" users have knowledge of).
However, if you really just want to change that login message (obviously an unsupported change);
The standard Clarity message is “CMN-01002: Enter a valid user name and password. Note that the password is case-sensitive.”
This text is contained in the file “CLARITY_HOME\resource\messages_en.properties” – the message appears twice in that file , so change both!