Closing it off may very well keep the mongols out... But it won't let new and interesting people in... Plus, I get the impression that ya'll enjoy stomping on them, but I could easily be wrong.
EDIT: Ah... Perhaps until they're approved by a moderator, or by vote or something, they're locked off to posting in a specific portion of the forums. The rules are simple. Survive the trolls, impress some people with your input, and you find your way out of the maze, over the wall, through the gateless barrier, whatever retarded metaphor you want to use. Fail, and you remain troll food until you either give up, or adapt... This way, retarded mongols will be kept somewhere where they cannot do much damage, could still auto-approve accounts, and people could troll noobs to their heart's content...
Though... I have no idea how difficult it would be to alter the forums in such a way, so I'm just talking out of my ass here... Might be stupid, I don't know... When you say a "great wall", I really have difficulty understanding what you mean, or how you intend to implement it. A multiple choice test of some sort, perhaps?