You've provided them with a username, under which you post, comment and view content. This is enough to identify you as an entity in the system and what it is you're doing. If what you're doing, based on heuristics and what you publish is having a bad effect on "the network", you can be blocked/stopped/warned.

I'm saying HN do anything of this, but I doubt they only look at your IP when you're interacting with the service.