Most Usenet clients would filter on the overview headers, which has the advantage that the messages did not have to be downloaded first, so it's much faster (these days that might not be such a issue), but the disadvantage that all a spammer/troll has to do (and did do) was change the From address to bypass the filter.
With Nfilter it was possible to filter on anything, for example crossposts (commonplace with trolls and spammers), NNTP-Posting-Host or even the Path, so a bad Usenet site with nothing but spam or trolls could be completely filtered out or flagged as junk.
The problem is nobody wanted to make even the small effort of running Nfilter (I tried repeatedly to get people to use Nfilter when groups were invaded by trolls but they preferred to just mindlessly continue complaining about it instead).
The author of Nfilter gave up development in the early 2000s, but its site is on Wayback Machine, from where it can be downloaded from its archived page (whether it works on anything later than Windows 95/NT I don't know), along with its source code:
So even without individual users running INN (which seems a bit like overkill to me), users can and always could filter out every type of abusive garbage very easily. They just have to want to do it.