Block User Automatically

There is a particular past username that somehow the hackers know about. I have three issues:

1) How do they know about this user at all since it has been deleted.
2) I know that a login attempt by this user is malicious – how do I automatically block them?
3) I had modified my htaccess file, but it seems they are still able to determine the usernames as one valid user gets spurious login attempts as well

Thanks much! Great product!
Steve

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