> spam was unheard of

Eh, not exactly ;) There was (and probably still is where needed) an elaborate system of killfiles being exchanged among trusted nntp servers. Also, you might want to look up the origin of the word spam:

> The first time that the word ‘spam’ was used in this sense actually arose from an innocent-enough affair. In 1993, Usenet administrator Richard Depew was responding to a discussion group, but he accidentally posted 200 duplicate responses to the board.

(from https://www.mailcleaner.net/blog/spam-world-news/whats-the-o...)