[FufLa]

The ONLY thing that could possibly work is having a global account system that servers use instead of making their own registration/login scripts. That way an user with an account could have his own unique identifier that is given to the server on login and the server uses that unique id to store data regarding the player. Of course you could always use it as a 2nd account system next to your own one but I doubt many users like logging into dozens of accounts.

On every login certain data could be synced with the global account system, like the current IP (that way the global account system would have a list of IPs used by the player) and a ban would be pretty effective once the player actively played on a server and got a lot of his dynamic IPs stored on the global account system.

This is the ONLY thing I can think of. Yet again you dont know if it works out (lag sending HTTP requests? I never tried the HTTP features out so I dont know if they can cause lag if the server the request is sent to is busy or not) and some server owners might not want to rely on a global account system that can render your server unplayable once it is down/not accessible.
A Filterscript could be created and you define how you want to participate, whether you want to use the global account login/registration and work with the unique id (which in fact would work like a normal account system) or if you only want to use the banlist that is provided by the system. But the banlist would also depend on quite a few servers using the global account system.

Sorry for the long post, just wrote down what I think is the only proper solution to having a global banlist.

SHORT: Global account system that stores all IPs used by the player and bans those once the player is banned. Servers use the global account system instead of having their own ones (or they just use it as a 2nd account you log in).