A close cousin of Lemmy is Mastodon. If you consider Lemmy a federated version of Reddit, then Mastodon is a federated version of Twitter.
The largest Mastodon server is probably Truth Social, on which former president Trump posts his messages after being banned from Twitter.
Truth Social uses the same protocol as Mastodon of Lemmy: ActivityPub. The difference: the Truth Social administrators blocked the Truth Social server from sending out messages to or receiving messages from other servers. So it's a private Mastodon.
Bottom line: if you run your own Lemmy server you can block whatever server you want or none at all. And others can block your server if they want. If you create ab account at somebody else's Lemmy server, the administrator can decide to block other Lemmy servers.
If you use a Mastodon account, it's very easy to migrate to another server including your followers. Lemmy accounts do not appear to offer that functionality (yet?), but I expect a migration tool will be created in the future. So if an administrator decides to block another Lemmy server, but you don't like that, you might easily move to another server. As of yet, you can't however and need to create an account on another Lemmy server.