Usenet killfiles are not "totally reliable". Nym shifting has always been a thing, even before Google Groups-based commercial mass spamming using constantly changing From: lines industrialized the problem. Killfiles also do nothing for people quoting the person you are trying to ignore, unless you use a thread-based killfile, which of course means you won't see a lot of non-killfiled people's comments.

At the end of the day, there is no satisfactory solution to the problem of warped and damaged online personalities other than actually preventing them from being online, which of course has its own difficulties and consequences.