By the end of September, kbin.social will receive an update that will address some of the issues. For now, I'm banning spam accounts, and registration is disabled. Additionally, external links are marked as "nofollow." I just want to emphasize that this is not solely a kbin issue - I had to manually remove spam posts from Lemmy instances as well because moderation didn't federate, which doesn't happen with, for example, Mastodon.
For now, the admin of your instance can choose to defederate or remove communities, as the admin of lemmy.world did. Once I've dealt with the update, I will reach out to other admins and discuss the possibility of restoring valuable posts.