They really didn't. And they certainly don't work now; just look at how completely overrun with spam Usenet is these days.

That's because all the humans moved to modern tools, but spam scales very easily.

There are a few places that I have visited that are now rarely used by humans, yet the activity of the spammers/bots is still alive and well to the point it is litterally the only traffic. To the obvious point, that you have to seriously wonder if the maintainers of the sites/platforms are not in on it.

What worked for Usenet was contacting the "sysop" of the node the user was posting from, usually upenn, and getting them to have a chat with the user and/or ban them. If their admin wasn't responsive, contact the upstream of their site.

Decentralized policing of users and having one responsible "moderator" per a few hundred users.

Then the internet was opened for commercial use and we got Eternal September and Canter and Siegel.

It worked for a while for the single user.

However with growth there were more bad users and misbehaving users still could give a bad impression to new people in a group, thus limiting acquiring new users.

I’m curious, How were those killfiles different than muting or blocking on Twitter? Usenet was a little before my time.

It's like muting users. Or muting messages by topic. Essentially a more advanced and useful technology than any of the social networks offer today.

Twitter allows you to mute users, keywords, and I believe specific threads. Killfiles weren't more advanced than that.

That’s just asking users to moderate themselves.