A global ban list could only work if there was regular evidence to support the need for the user to be banned. Despite different rules for different servers as explained above, not every admin is trustworthy in themselves. I could imagine a lot of threats, bias, and corruption by admins who ban people they just don't like.
I still think a reporting system is best. Say for example, if the user is reported more than 5 times for serious reasons, they could be put into the system. Either way, a global ban system would need to be operated by LU developers as far as I'm concerned. People can't be trusted managing it themselves.
In this case, letting people choose who they want banned on their own servers is good enough for most players and servers. KAS is a bit of an exception because he indiscriminately wants to wreck servers and prey on players. Most people don't act this oddly.