HRoth - 2:45 pm on Dec 16, 2012 (gmt 0)
I think for something to be Fair Use, the origin of the quote has to be noted in some way. I'm thinking these three sentences were probably not even in quotation marks, so that would not be Fair Use. I've experienced this sort of thing quite a lot with my site and with participants in forums all over just copying stuff from my site and posting it in a forum post as if they had written it. That said, I don't bother to do a DMCA on something like that.
If it were me, I would simply have removed the offending post, not the thread, and let the poster know that it violated Fair Use, if it did, and explained how. I have seen forums where they make a sticky about Fair Use and where as a result there is very little content theft. I would not play any silly games where I hid that post or thread or whatnot. Believe me, if I find someone using my content behind a subscription wall, I am going after them hammer and tong. I suspect your nutjob would do the same, and there is no way you can ever be sure s/he would be kept out.