Just to be clear, I don't mean to say that I don't reply to anybody, ever. Far from it. I enjoy a good conversation - *so long as it's some combination of productive, interesting, polite, professional, etc.*. I was trying to say that I'm more willing now to not reply when it's a response that is clearly just trying to start an argument for the sake of starting an argument, or somebody who is rude / disrespectful / etc.

I reply to people all the time, especially on HN where the S/N ratio tends to be a bit higher.

Indeed. When the conversation goes for example in the direction of: 'i am right, you are wrong', it is best to categorize as noise not begging a reaction.