If you change your home feed to subscribed, then you don’t have to block anything, you just have to not subscribe to those communities.

The word you’re looking for is “copula”, and it’s actually 「です」(desu)

It’s actually 「は」, and… kind of. It marks the topic, which is sort of the thing the conversation is generally about, which typically is the subject of each sentence, but not necessarily. It’s kinda hard to explain it well since it doesn’t really map cleanly onto any grammatical feature in english.