Options are move the FC to a world that's not full, or just don't be in the same FC as all of your friends but you can still do all battle content with them and chat by cross-world linkshell. They don't lock worlds to annoy people, it's just battling against physical laws that say one world/server can't have infinite capacity. It wouldn't be worth leaving a world permanently open and unrestricted because they have to provide a service which wouldn't be possible if they allow players to crash the server and bring the game offline.