Nothing has changed since Jerry Pournelle wrote 40 years ago when discussing online forums:

>I noticed something: most of the irritation came from a handful of people, sometimes only one or two. If I could only ignore them, the computer conferences were still valuable. Alas, it's not always easy to do.

This is what killed Usenet,[1] which 40 years ago offered much of the virtues of Reddit in decentralized form. The network's design has several flaws, most importantly no way for any central authority to completely delete posts (admins in moderated groups can only approve posts), since back in the late 1970s Usenet's designers expected that everyone with the werewithal to participate online would meet a minimum standard of behavior. Usenet has always had a spam problem, but as usage of the network declined as the rest of the Internet grew, spam's relative proportion of the overall traffic grew.

That said, there are server- and client-side anti-spam tools of varying effectiveness. A related but bigger problem for Usenet is people with actual mental illness (kstrauser mentioned one); think "50 year olds with undiagnosed autism". Usenet is such a niche network nowadays that there has to be meaningful motivation to participate, and if the motivation is not a sincere interest in the subject it's, in my experience, going to be people with very troubled personal lives which their online behavior reflects. Again, as overall traffic declined, their relative contribution and visibility grew. This, not spam, is what has mostly killed Usenet.

[1] I am talking about traditional non-binary Usenet here