Being kind to them is completely wasted effort.
Replying to them is also wasted effort as they won't be persuaded.
However leaving bullshit unchallenged might make trusting bystanders believe that this is actually the truth.
1. xkcd 386: Someone is wrong on the Internet: <https://xkcd.com/386/>
2. Woozle's Epistemic Paradox: as epistemic systems become more prominently and widely used, they also become more attractive targets to those who would choose to manipulate them. See: <https://web.archive.org/web/20230606193813/https://old.reddi...>
3. More generally, all informational channels become battlegrounds, as noted by von Clausewitz, Sun Tzu, numerous evolutionary biologists, and others. I've been slowly making my way through Jeremy Campbell's The Liar's Tale which is a big-picture look at this.
4. HN really isn't optimised around truth but conversation, and more specifically sustainable conversation, grounded in intellectual curiosity. There are numerous topics on which HN really is unable to have meaningful discussion (perennial ethnic conflicts are amongst these, as are other political hot topics), and its general tone-policing and penalisation of high-tension topics tends strongly to a status quo bias. (I've criticised HN for this often, despite an increasing awareness and appreciation of why those rules exist, see e.g., thread here: <https://news.ycombinator.com/item?id=39023516>.)
5. Reporting blatant trolling and suspect motivations to HN mods does often work. Email to hn@ycombinator.com and link directly to the offending content and/or user, with a clear but succinct description of the problem.
6. Voting (up or down) and flagging also have their place. For sufficiently contentious threads this may well lead to something of a high-attrition zone, but often the really egregious crud does sink to the bottom. I find that higher-rated comments tend to be more anodyne than insightful, though occasionally truth does out.
7. I've found that rather than direct engagement, either supporting a counterthread, or writing your own well-reasoned and well-supported counter-thread, is often suprisingly effective. Remember that yours is always the last comment when you write it, though a thread may well have additional life. Sometimes my late efforts prove far more successful than I'd expected, and often I'll see that others have succeeded where I've either failed or failed to try. And again, supporting others' salient and productive engagement even where you don't have time or energy to contribute is highly underappreciated.
8. You don't have to attend every fight you're invited to.
9. Truth is not a popularity contest. Voting systems ultimately don't select for truth or importance, and expecting that from sites such as HN or Reddit will prove disappointing.
10. The meaningful audience is typically not who you're responding to directly, but the overwhelmingly silent majority reading rather than contributing to discussion.