It is sometimes necessary to temporarily disable an account, without removing it. For example, the user might not have paid his fees, or the system administrator may suspect that a cracker has got the password of that account.
The best way to disable an account is to change its shell into a special program that just prints a message. This way, whoever tries to log into the account, will fail, and will know why. The message can tell the user to contact the system administrator so that any problems may be dealt with.
It would also be possible to change the username or password to something else, but then the user won't know what is going on. Confused users mean more work. 
A simple way to create the special programs is to write `tail scripts':
#!/usr/bin/tail +2 This account has been closed due to a security breach. Please call 555-1234 and wait for the men in black to arrive.
If user billg is suspected of a security breach, the system administrator would do something like this:
# chsh -s /usr/local/lib/no-login/security billg # su - tester This account has been closed due to a security breach. Please call 555-1234 and wait for the men in black to arrive. #
Tail scripts should be kept in a separate directory, so that their names don't interfere with normal user commands.
But they can be so fun, if you're a BOFH.