The "Ban usernames" page does not actually ban usernames. It bans existing user accounts, and if those get deleted the usernames can be used to register again. "Disallow usernames" applies to usernames.
Suggestion is to rename "Ban usernames" to "Ban users" or maybe "Ban users/IPs".
Support topic: http://www.phpbb.com/community/viewtopic.php?f=46&t=2128917