It's interesting, because there are sources on the internet that catalogue a taxonomy for all manner of fallacious arguments (Wikipedia is one obvious example), but there doesn't seem to be a name for claiming that internet posts written by the people in set A are hypocritical because they presumably-contradict either 1) posts by people in set B happened to appear on the same website, or 2) some hand-waving characterization of the zeitgeist of the site's denizens.

This is clearly fallacious, I don't know why people keep using this technique, and we really should give it a name so that it can be referred to by shorthand. (Any suggestions?)

If you happen to find a person in the set A intersect B, then you've got a case against that individual. But nobody seems to go to that much effort.