Ban him and ostracize him socially.

You're right. It seems silly to say that nothing, with emphasis, will stop Will when banning him and any obvious sockpuppets hasn't even been tried. (This isn't particularly advocating that course of action, just agreeing that Luke's prediction is absurd.)

"Ostracize" does not work well online. You don't get direct feedback on how many people read what. (Even the downvotes are evidence that someone did read the comment, and expended some of their energy to downvote it -- which supposedly is part of what the trolls want.)

There is no online equivalent of a group turning their backs on someone in ice-cold silence. Just "not answering" is not the same thing... that happens to many normal comments too.

I strongly disagree. As far as I am aware, there is no contract between Will Newsome and LessWrong/the SI/FHI/CFAR that states that he is entitled to do whatever he likes. In fact, per community norms, the opposite is true. The claim that he is a SuperTroll seems to be self-evidently true, and is almost certainly not libel, as Will Newsome has done his best to encourage the idea that he is a troll, and possibly even began this view. (Consent is usually considered a defense against libel.) Newsome also seems to have disruptive intent- he's explicitly stated that he's trying to burn his credibility as fast as possible. So, the mods have fairly solid reasons for moving against him- at present rates, it'll take ~27 months for him to burn through the rest of his karma, which is far too long.