[kh99]

In includes/functions_login.php there's a call to vbmail, I think if you comment it out that should do it.

If you don't want to edit any files, you could try making a new plugin using hook location mail_send with this code:

if (substr($subject, 25) === 'Failed Login Notification')
{
    return false;
}

Of course that assumes that you haven't changed that phrase. (BTW, I haven't tried any of this.)

(Edit: ...changed if to ===).