Edit the sshd_config file, the location will sometimes be different depend on Linux distribution, but it’s usually in /etc/ssh/.
Open the file up while logged on as root:
vi /etc/ssh/sshd_config
Insert a line:
DenyUsers username1 username2 username3 username4
Referring to #man sshd_config:
DenyUsers
separated by spaces. Login is disallowed for user names that
match one of the patterns. â*â and â?â can be used as wildcards
in the patterns. Only user names are valid; a numerical user ID
is not recognized. By default, login is allowed for all users.
If the pattern takes the form USER@HOST then USER and HOST are
separately checked, restricting logins to particular users from
particular hosts.
Save it and restart SSH services. Basically username1, username2, username3 & username4 SSH login is disallowed.
/etc/init.d/sshd restart
Reference: http://www.planetmy.com/blog/
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source:http://linuxpoison.blogspot.com/2008/08/13578175716347.html