Allow / Block ssh / sftp for specific Users / Group

Allow ssh / sftp to selected users
If you want to give ssh / sftp privileges to certain users, here is the solution.
Add following line in the file /etc/sshd_config

AllowUsers username1 username2 username3

If you want to allow certain groups only, add following line in /etc/sshd_config

AllowGroups username1 username2 username3

Restart service ssh.

Block ssh / sftp for selected users
If you want to restrict certain users to access ssh / sftp, here is the solution.
Add following line in the file /etc/sshd_config

DenyUsers username1 username2 username3

If you want to block certain groups only, add following line in /etc/sshd_config

DenyGroups username1 username2 username3

Restart service ssh.