Do, what, specifically? How will they affect that your instance shows you?
Its more like email lists blocking people from other email lists. If there is a massive email list that has continually and specifically coordinated to destroy or consume other email lists and spent massive resources learning specifically how to do this via social manipulation, yes, I would think blocking people from that email list is a very good idea ^.^
Should a listserv block people who are subscribed to another listserv then?
Perhaps if it wasn’t already corporate agglomerated, this wouldn’t be so true. But fediverse isn’t email, we have easier indicators for abuse because most content is public and we can guesstimate how much of an instance is “real” users ^.^
An email is a message from a user at a domain. A fediverse post is a message from a user at a domain.
Content is public, but to a big email provider, it’s not much more data. Gmail filters based on identical-looking messages from an “unestablished” domain. If you came up with a way to filter spam on the fediverse, it would likely look very similar.
If Mastodon/Lemmy/whatever picks up critical mass, I can guarantee you there will be a shit ton of spam, misinformation, disinformation, and scammy nonesense coming from a long tail of instances. Much of the garbage will, thanks to large language models, look pretty human, too. The only real roadblock to it will be defederation from “unestablished” instances and even that will be unreliable at best.
There really isn’t a good solution to it, at least one that isn’t invasive in ways we won’t like.