That's not an exploit, in fact there are several FATEs that require you to kill certain enemies to end it. or turn in a certain number of items to end it. all of them are timed as well so it isn't "infinite" just because you probably couldn't get a decent party for it or you want people to spend 10 seconds on every over crowded FATE does not mean it needs fixing, you're like the guy who sees everyone having a good time and you chunk a couple of smoke bombs in the middle of them.