Users can be banned by username, IP address or email address. The "Ban usernames" page does exactly what it advertises, bans by usernames. I see no reason to change something that isn't broken. One (or even a handful) of support topics from new users who have that "duh" moment when they delete previously banned usernames is not significant justification to change what is not confusing for the majority. Change should be driven by the majority for the benefit of the majority not a handful of inexperienced users who want phpbb to behave like something else or who cannot be bothered to RTFM.
UI changes to mature software should be limited to critical bug fixes or the introduction of new features. Constant tweaking of the UI just alienates existing users because even small changes require attention be spent adapting to those changes (the unfamiliarity factor) which is time they didn't have to spend before. This generally results in resistance updating to new versions which is the opposite of what you want here. When it comes to UI change management, if it ain't broke don't fix it. How do you tell if it's broke? When you are getting the same workflows question from dozens or hundreds of people over an over. Basically your top ten FAQ's in support will tell you where you biggest UI problems are and where to focus your resources. Can almost everything be "improved", sure but just because it can does not mean it should be at the present time. The 3.0 branch is four years old now, 3.1 or 3.2 is where you can get away with significant UI changes without alienating the existing user base since a major version change is expected to have a lot of UI changes. -1 on this proposal from me.