Preventing Login to a Machine
To prevent users from logging into a machine, create an override entry with an invalid string in the passwd field. Because the passwd field contains an encrypted password, any character string that is not exactly 13 characters in length can be used as an invalid password. For example, the following entry in the passwd_override file supplies exclude as a password. This string of less than 13 characters prevents members of the group that is identified by a UNIX ID of 25 from logging in.
:exclude::25:::