Some thoughts —

The original "Eternal September" (on Usenet) wasn't an influx of abusers. It was an influx of new users who didn't know how to do things properly yet.

Most of the new users were from the America Online (AOL) private service, and known as "AOLers". (As it happens, I joined Usenet around the same time, but from a local dial-up Unix BBS in the Washington DC area.)

The AOLers didn't know which aspects of the service as they saw it were due to the AOL custom client software, which were due to the AOL local server, which were due to the newsgroup (forum) they were looking at, and which were due to the global Usenet consensus. So when they had a problem, they didn't know where to address that problem. They complained on public newsgroups about UI issues with their local client, because they didn't know what was what.

And the existing users didn't have the time or capacity to help them. The AOLers were added to Usenet en-masse without preparation. Nobody had signed up to help them. The AOLers were accustomed to AOL chat rooms that had staff helpers and moderators; most of Usenet did not have any — just regularly-posted FAQ documents, which the AOLers did not know to look for, and grouchy users who angrily told them to read the goddamn FAQ before posting.

Another consequence of the influx of new folks was that Usenet suddenly just had a lot more people. This made it a tasty target for commercial spammers and other abusers; which led to the eventual spampocalypse and a lot of people abandoning Usenet for web forums or other services.

It wasn't long into Eternal September that the hardcore abusers showed up, though. That, I think, is the harder problem to deal with.

"Good" Usenet servers did not reliably disconnect themselves from the servers that were accepting and forwarding spam. It was not generally acknowledged that a good server needs to block bad servers: the free-speech ideal was assumed to mean "accept anything from anyone; let the client decide what to filter out" — which meant that new users who had not written any filters necessarily saw all the spam.

And because nothing was secured by strong encryption, forgery was rampant; with a little cleverness, anyone could pretend to be anyone from any server.

There were many, many efforts to fix the spam problem. Unfortunately, as things turned out, it wasn't enough. Eventually folks noticed that the NNTP facility offered by their ISPs was a great means for sharing pirated porn ....