You can use AllowUsers / AllowGroups if you have only a few users/groups that are allowed to login via ssh or DenyUsers / DenyGroups if you have only a few users/groups that are not allowed to login. Note that this only restricts login via ssh, other ways of login (console, ftp, …) are still possible. You need to add these options to your /etc/ssh/sshd_config file for most ssh installations. If you have set the login shell to /bin/false you can use su -s /bin/bash user (replace /bin/bash with the shell of your choice)