There was an issue where lemmy.ml refused connections from kbin instances but the admins of that instance fixed the issue yesterday. At no point did true defederation occur.
I don't recall us defederating from them. Maybe try moving to beehaw.org, the definitely defederated from lemmy.ml and they have stricter moderation standards overall.
Probably a good idea. You could find an instance that suites your needs or run one yourself as mentioned in other comments here. Instead you would rather be toxic and while I dont speak for the others, it’s probably not welcome.
I have tried banning the instance but that no longer works.
If with this you mean you blocked the domain, well, for some reason non-link posts use the kbin domain regardless of which instance they're from. So blocking the domain a thread is from won't stop you from seeing that thread. In theory, idk how it works in practice, but what you're experiencing seems to confirm this.