If you haven't heard, sadly, #ChirpSocial, one of the popular #ActivityPub “groups” platform, is shutting down “probably” on February 29th.
In an email they sent to admins, the owner and developer can no longer support https://chirp.social financially as they failed to find a new job after they were laid off by #Google last year.
This reminds us the importance of having a built-in groups feature, and one where the groups feature actually federates.
Back in 2008, when the #Fediverse was born, we did have a built-in federated groups in #Laconica / #StatusNet (today known as #GNUsocial). We used bang (!) instead of at (@). A built-in groups feature is more stable as established instances can host them.
Today, we have #Friendica and #Hubzilla (as well as #Streams-based instances) to fill in that, as groups is a built-in feature in those software products. It's just a matter of finding an instance that's open to hosting groups for any topic for the ActivityPub protocol.
That said, any Friendica, Hubzilla, Streams-based instances you suggest for groups?
It will depend on the platform. For example, #Friendica (and to an extent #Hubzilla and #Streams), the owner of the group (and anyone else given moderation access) can block accounts. There is also chirp.social which can also block accounts.
Then there is #GNUsocial, which is a rebranded #StatusNet itself a rebranded #Laconica (the first #Fediverse software, c. 2008) have built-in groups feature; which IIRC, can also block users if needed.
Personally, services like Guppe really need to add moderation features, otherwise, what you just described will more likely happen.