Handle the situation when a user doesn't exists

[kukukk]

What is the recommended way to show an error page when the user is not allowed to login?

I need to handle the situation when a user doesn't exists at the SP side. I don't want to automatically create a new user. I want to show an error page/message instead. The documentation just mentions that
if it does not exist create it and go on or show an error message
But there is no example on how to do that.