Each region displays its banlist on the administration page, could the same be done for a nation? Each one could have a tab, much like the administration or history tabs on a regional page and every region that a nation is banned from could be listed. This would be useful for regions to spot troublemakers straight away and avoid incidents or trolling, flaming, flamebaiting and spamming.
I don't know if that would be a lot of code, but it was manageable for regional pages.