Comment by maxbond

I don't have any proprietary knowledge of how HN does voting ring detection, but to offer an intuition about how it might work in the scenario proposed by GP, this voting ring would be detectable because their flags are highly correlated and clustered together in time. The more stories they attempt to flag down (successfully or otherwise), the more obvious the pattern will be.

I'm sure you're already thinking of ways to bypass that, and yes what you're thinking will probably work, it's a game of cat and mouse and no one technique will be sufficient or work forever. (See also https://en.wikipedia.org/wiki/Defense_in_depth_(computing) )