Issues like this are why I'm not excited about Mastodon. The designer didn't think through possible harms.
And like a lot of open source projects, its conceptual anchor was an existing product/service (in this case, mid-2010s Twitter).
I think this is going to be a big problem. :-/ Mastodon hasn’t really been set up to deal with large scale coordinated harassment - and some instances are going to be bad at moderation (some intentionally, some unintentionally).