Additional thanks to Lemmy clients that allow keyword filters unlike the default Lemmy UI
Regarding the post title: Lemmy itself allows you to block communities, instances, and users. But it does not support filtering by keyword, e.g. filtering out any post with the word "platypus" in the title regardless of where it was posted or by whom. With help from the comments below, here are some third party Lemmy clients that have their own implementation of keyword filtering:
Instance blocking despite all the promises turns out to be mostly a lie, and has actually gotten weaker with time. Although we were told to wait and then this feature would solve our personal defederation needs, the opposite turned out to be the case. On 0.19.3 it at least used to prevent notifications (confusingly enough), then on 0.19.5 it no longer does so.
I still see every comment, they can reply to me, I can reply to them, and the only thing it ends up doing is hide communities - including auto-name-expansion with the exclamation mark. I wanted an actual "block" of an "instance", but apparently I must move to another instance to get that:-(.