How to block specific username from ssh?

Put DenyUsers user1 user2 user3 in /etc/ssh/sshd_config

Artyom answer is correct. I'd just like to point that there is also the possibility to opt for a 'white-list' approach instead of the 'black-list' one by putting a line like this in /etc/ssh/sshd_config:

AllowUsers AuthorizedUser1 AuthorizedUser2

and reloading ssh service (service ssh restart)

Then every other user will be denied ssh access (be careful not to lock yourself out ;-) )