USENET never really solved spam. I remember the statistic that at one point one third of usenet traffic was spam and another third was spam cancels. Here's what people felt at the time: https://www.eyrie.org/~eagle/writing/rant.html

(the "cancel" message was hilarious since when invented it was unauthenticated, i.e. anyone could delete any post on any group in USENET! This had to be fixed:

https://www.templetons.com/usenet-format/cancel.html )

Also found https://www.gdargaud.net/Hack/NoSpam.html which is a great little time capsule site..