Usernames do not exist, lol. You just pick a name when you join a Server, and the Server can then deny any duplicate Screen-Names. Any Host can ofcourse use those Screen-Names in Commands when it comes to things like Moderation. There would be a lot of utility to make it easy for Humans to use.
The Problem with a Ban List is, if you do not pay for an Account, it is completely useless. Anyone can just spin up a new Account on the fly if you dont use a Whitelist.
What COULD be done is a basic Ban List for certain Computers, MAC Addresses, IPs etc., but a truly malicious Actor could just toggle some things in a fork of the Open Source Launcher and suddenly his reported Data is different.
Heck I know how to change my MAC Address on the fly, simply because WiFi Portals tend to detect that for the sake of Bandwidth Caps…