At the risk of taking this too meta... there's something interesting here about online communities like HN.

In everyday human interactions, we have a lot of contextual and social cues about who is more experienced, respected, expert, even who is older; in short, all kinds of social hierarchical information that guides and constrains our behavior. Online, especially in a community of sufficient size that most interactions are between strangers, as here, we lack these cues and must approach each other as equals, or at least as unknowns. It's very democratizing, and sometimes it's incredibly aggravating.

There are things a professor can say to a student that a student can't say to a professor. The professor can police students' behavior and ask a student to leave. So we would not expect this kind of "aggressive defense of ignorance" in a classroom, because there is someone there who is responsible for creating a different environment, as part of an effective, millenia-old tradition of inquiry.

The zen master can whack the novice with a stick. Sometimes leading to enlightenment, sometimes just to the master getting some peace.

As with the professor, it's the relationship and the environment that makes this acceptable. You won't get far by telling strangers on the street that they are woefully uninformed, or by whacking them with a stick. Online, we're essentially all strangers on the street.

Certain basic life skills and attitudes can be communicated and acquired almost immediately when you can get whacked with a stick that can hardly be acquired online at all. Online communities seem to be a terrible place to learn humility, for example.

In response to these realities of online engagement, there are community-level and individual-level approaches. Leaving the former aside, as an individual there are two things I find helpful, one is detachment and the other is to remember the audience.

The "wrong on the internet" compulsion maybe comes primarily from expectations developed offline, where we can talk sense into people, generally within some institution that facilitates this (school, church, work, etc) and which generally doesn't exist online. This can be annoying. Perhaps it's this annoyance that largely leads to endless online arguments. Sometimes you want to communicate to someone, gently but unmistakeably, that you know more about the topic at hand than they are likely to learn in the next ten years. This is likely something that would be communicated automatically and invisibly by environment and context offline, but is almost impossible to communicate at all online. Online, the distinguished biologist and the earnest creationist high school student appear to have equal weight, especially to the high school student. Sometimes you feel the need to communicate to someone that their ignorance of the topic is matched only by their ignorance of their own ignorance, but you can't. You wouldn't be heard, the environment doesn't support it, and anyway it looks bad. If we're all as equals, or at least unknowns, the next person who copies your strong style, more likely than not, will lack the experience that justifies it.

Remembering the audience means that any interaction in a public forum is more likely to influence bystanders than the one directly addressed. It's like a debate, in which debaters address each other but actually aim to persuade the audience. Unfortunately this usually means it's all rhetoric and favors shallow attention-getting over deep discussion and exploration, but that's another topic. Anyway, you can reply to the person but aim more to persuade the audience, which in general is bigger and more likely to be swayed by your reasons than someone who is already arguing against them.

Beyond promoting your position, trying to encourage better discourse, from the audience, rather than the person addressed, seems to help.