Limiting user accounts that can connect remotely via SSH...

1) Installing SSH

    $ sudo apt-get install openssh-server

2) Backup current ssh server configuration file

    $ sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.backup

3) Edit the configuration file

    $ sudo gedit /etc/ssh/sshd_config

4) Change the parameter to disallows the root user from connecting

    PermitRootLogin no

5) Add the parameter and specify the usernames (space separated) that can connect remotely.

    AllowUsers <user1> <user2> ...

6) To deny

    DenyUsers <user1> <User2> ...

7) If you enable the OpenSSH server and you do not wish to enable any remote connections, you may add

    AllowUsers nosuchuserhere

8) Restart the service

    $ sudo /etc/init.d/ssh restart