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.
Edit: sorry this took me so long. Here are my thoughts based on past experience:
It's not helpful to look at other accounts as "propagandists", "trolls", "bots" (or "shills", "astroturfers" or all the other similar terms internet commenters use), because there's no reliable way to distinguish between someone who is arguing for views they earnestly hold vs. someone who might be engaged in a propaganda campaign. The majority of commenters are sincerely arguing for what they sincerely think. A few may be doing something more sinister—but most people don't do such things, and there's no way to pick out the ones who are. Trying to pick them out will drive you crazy. Moreover, it doesn't matter in the end, because everyone is ultimately working with the same tool: arguing things in comments. Even the propagandists can't do more than that.
When people feel like someone else is a troll, propagandist, or bad faith actor, it's usually because that person's views are so distant from your own that it seems like nobody could possibly hold them in good faith. In reality, what's usually going on is that people on the internet underestimate how different each other's backgrounds are. It feels like you're talking to someone who is either insane or lying, but most likely you're talking to someone whose background is so different from yours that it's hard for the two of you to relate to each other. I wrote about this last year, if anyone is interested: https://news.ycombinator.com/item?id=35932851.
I think the only way to handle this is by responding to bad arguments with better arguments and to false information with correct information—and to do this as neutrally as possible. Focus on clear information, and try not to let your feelings turn into aggression toward the other person. This is not easy, but it's in your interest to do it, because when commenters get aggressive with each other, fair-minded readers recoil.
For extra influence, if you can manage it: look for a way to connect with the other person, acknowledge some aspect of what they're saying, and implicitly make it clear that you're not trying to defeat or destroy them, but rather to understand. This is a big multiplier on how persuasive your comments become.
As for leaving bullshit unchallenged, I know it's hard to walk away from a thread that one feels is dominated by falsehood and distortion, but walking away is sometimes the most effective thing you can do. Here are a few thoughts which I try to remember in such situations:
(1) The internet is wrong about approximately everything. You can't change that, and you'll burn out trying.
(2) The one who walks away first usually comes across as stronger.
(3) Other people are not that different from you. When someone seems crazily wrong, they're most likely not bad or evil, but ignorant: they don't know what you know because they haven't experienced what you've experienced. For this reason, sharing your personal experience is probably the most effective thing you can do.
(4) When other people say things that produce strong feelings, try to let the feelings run their course in you before coming back to react. This is painful and hard, but it's in your interest.
That's... not 100% honest, I guess, but at least it makes for easier to read and easier to handle conversations.
Conversely, arguing on HN has definitely helped me find the words to explain my own thinking to myself, so there's something in flogging a dead horse, sometimes.