If you think you see evidence of abuse, bring it to our attention so we can look into it: hn@ycombinator.com. It's against the site guidelines to go on about this in in the threads without evidence, so please don't.
From years of looking at this data, I can tell you that beliefs about "brigading", "astroturfing", and so on almost always turn out to be imaginary. What's really happening is simply that the community is divided on divisive topics.
I'd be really interested in hearing of HN moderators have observed that kind of thing. It definitely feels like it from time to time, but it could also just be that the topics are inherently polarizing, so only the loudest voices on each side come out.
But then again, social media brigaiding in the interest of shaping public opinion is very widespread, even lucrative business.