I ran the tech side of the most popular independent ISP in Chicago (I guess they were mostly all independent back then) in the mid-late 1990s, and Usenet was the biggest nightmare we had to deal with. We were solid at it, too (Freenix-ranked, independently worked out the INN history cache, &c). Nothing we did had more fussy hardware associated with it.
The problem for us wasn't spammers; it was binaries. That's what killed Usenet.
My experience in the 1990s was that that was a very excellent way to get all the Usenet users in your userbase to switch to some other ISP, which in turn meant there was little reason to waste all the energy on Usenet if you were going to do that, which contributed to the centralization of Usenet, a system that did not make sense once centralized and that subsequently collapsed into a shitty P2P piracy network.
Even back then, though, it was always under attack by spammers. https://en.wikipedia.org/wiki/Laurence_Canter_and_Martha_Sie...