I wonder if you could try to address this by limiting who can flag a given post. Even just doing it very naively and choosing, say, a fifth of your users for e

I wonder if you could try to address this by limiting who can flag a given post. Even just doing it very naively and choosing, say, a fifth of your users for each post and only giving them the option to flag it might help significantly. It would probably make it more difficult to motivate the members of these problematic groups to actually coordinate if the average expected result was the inability to participate. And you could do it in more sophisticated ways too, and form flag-capable subsets of users for each post based on estimates about their similarity, as well as any other metrics you come up with - such as selecting more "trustworthy" users more often. This would help gather a range of dissimilar opinions. If lots of dissimilar users are flagging some content, that seems like it should be a strong signal.