They've got a massive spam problem, and their mod actions do not federate out. So, while they may be cleaning up spam on their end, none of that federates to Lemmy and we get stuck cleaning up the mess in its entirety.
Have had good experiences with users from kbin.social, so it's not them that's the problem. Looking at the magazines on the Kbin side, they are cleaning up spam, so the mods/admins are on the ball. But without those cleanup actions federating out, they're still contributing to fediverse-wide spam that everyone else has to deal with.
Also, Kbin doesn't seem to have the concept of a registration application, only a captcha, which seems to be a very low barrier to entry for the spam bots that keep popping up there.
https://codeberg.org/Kbin/kbin-core/issues/570
Once that issue with Kbin core is addressed and mod actions from Kbin start federating, we will resume federation with kbin.social.
Update: We've re-federated but have removed all communities that are hosted on kbin.social
@Antik@lemmy.world suggested removing the Kbin communities to alleviate the spam vector. That will still allow Kbin users to interact with us (they were never the problem) while preventing spam coming from their communities where their mod actions don't propagate out. If any spam from Kbin comes through to a Lemmy community, mod actions can be taken that will be federated.
Thanks, Antik!