The banlist is a tool for founders and WA Delegates to keep nations they don't want in a region out of the region.
A founder (or UN delegate if delegate regional control is on) may eject a nation, sending them to the Rejected Realms. They are also put on the banlist, which prevents them from re-entering the region unless they are removed from the banlist.
Originally a delegate or founder would be able to keep as many nations as it wanted on the ban list (within certain rules, see: griefing). However, following many complaints of Francos Spain's alleged overuse of the banlist in the Pacific during the fall of 2003, the banlist was limited to 200 nations.
Also, in Warzones, the banlist is automatically cleared twice a day.
To view the banlist of a region, type in this URL. Replace the italic text below with a desired region name:
http://www.nationstates.net/page=region_control/region=(region name)