this really should be as easy as a database DELETE operation, why is it so difficult? guess the design for HN site is not "user-first". recalling facebook etc was impossible to remove your account but now it seems feasible and quick, though not sure if facebook/etc indeed flushes the deleted-account cleanly.
Hacker News doesn't use a database - data is stored in Arc Lisp closures in flat files. I'm not certain how it handles usernames, though (I've played around a bit with Anarki, but can't remember) but I suspect it would probably involve a lot of file parsing and editing, assuming each username is literally just copied into a field in each thread file.
Are you sure? What the word "closure" usually refers to isn't an externalizable object used for persistent storage.