Option 1: Close the borders. It's unfortunate that the best sort might be kept out, while its guaranteed the rest will be kept out. The best can found / join other sites, and LW can establish immigration policies after a while.

This isn't so ridiculous in short bursts. I know that Hacker News disables registration if/when they get large media attention to avoid a swathe of new only-mildly-interested users. A similar thing could happen here. (It might be enough to have an admin switch that just puts a display: hidden into the CSS for the "register" button; trivial inconveniences and all.)