This make me think that we should maintain a community curated blocklist in, for example, a Git repository.

There would be a few problems I can think of with this approach. The first one is who controls it? Whoever that is, you haven't solved the issue because now instead of only the instance with the user being able to federate the ban now only the maintainer of the git repo can update the ban list.

If you have many people able to update the repo, then the issue becomes a question of how do you trust all these people to never, ever, ever get it wrong? If you ban a user and opt to remove all their content (which you should, with spam), then if you are automating this you end up with the issue of if anyone screws up then how do you get someone's account unbanned on all those instances? How do you get all their content restored, which is a separate thing and Lemmy currently provides no good way to do this. How do you ensure there are no malicious people with control of the repo but also have enough instances involved to make it worthwhile?

There is a chat room where instance admins share details of spam accounts, and it's about the best we have for Lemmy at the moment (it works quite well, really, because everyone can be instantly notified but also make their own decisions about who to ban or if something is spam or allowed on their instance - because it's pretty common that things are not black and white).

I would honestly have expected something like this to already exist. I think it’s partly the purpose of Fediseer, but I’m not completely sure.

Fediseer has a similar purpose but it's a little different. So far we have been talking about spam accounts set up on various instances, and the time it takes for those mods and admins to remove the spam. But what happens if instead of someone setting up a spam account on an existing instance, they instead create their own instance purely for spamming other instances?

Fediseer provides a web of trust. An instance receives a guarantee from another instance. That instance then guarantees another instance. It creates a web of trust starting from some known good instances. Then if you wish you can choose to have your lemmy instance only federate with instances that have been guaranteed by another instance. Spam instances can't guarantee each other, because they need an instance that is already part of the web to guarantee them, and instances won't do that because they risk their own place in the web if they falsely guarantee another instances (say, if one instance keeps guaranteeing new instances that turn out to be spam, they will quickly lose their own guarantee).

Fediseer actually goes further than this, allowing instances to endorse or censure other instances and you can set up your instance to only federate with instances that haven't been censured or defederate from instances that others have censured for specific reasons (e.g. "hate speech", "racism", etc).

It's quite a cool tool but doesn't help the original discussion issue of spam accounts being set up on legitimate instances.