There's nothing preventing you from implementing your own social network with those rules.
Hacker News is a forum for business ideas and web-based technologies after all, built around an angel fund / venture capital slant.
But I'm trying to tell you that you'll end up relying upon user-based moderation for many cases: including simple cases like spam from new users or VPNs. Maybe even IP-based blocking as certain unsavory groups of people tend to cluster around certain IP addresses.
I responded to your statement that 'The _ONLY_ moderating action internet citizens have is moderation [...]'; I'm just pointing out that while that is true for twitter users at present, Twitter could experiment with other approaches if they felt like it. I'm not saying they must or they should, just that nothing prevents them. I responded in handwavey terms because your comment about 'internet citizens' seemed pretty open-ended.
But I'm trying to tell you that you'll end up relying upon user-based moderation for many cases
OK? That's the idea I was exploring. It worked to some extent on NNTP, where people would maintain 'killfiles' and different Usenet groups were more or less protective of their cohesion. I'm not suggesting it as some sort of magic bullet, just saying that centralized moderation is not the only possible approach.
And arguably not, because a huge number of NNTP servers were in fact, moderated. Especially as spam and warez started to appear on servers owned by public institutions (universities and the like), a huge chunk of USENET split into the alt.* unmoderated groups, and the other, centrally moderated groups.