From his website stallman.org:
Richard Stallman has cancer. Fortunately it is slow-growing and manageable follicular lymphona, so he will probably live many more years nonetheless. But he now has to be even more careful not to catch Covid-19.
Recent video of him speaking at GNU 40 Hacker Meeting. Screenshots of video stream.
You can’t, not effectively. This ain’t necessarily a federation problem though, centralised social media has the same problem. You can do the same on Reddit.
That’s not to say there aren’t things that can be done. Instances can agree to implement shared banlists that include known-abusive IPs or, if an abusive user has a habit of using a pattern in their evasion usernames, you can use a Regex to hunt them out.
On reddit the entire user database is in the hands of one company, so they might be able to tell whether two people are (likely) the same person. So I think that might actually be a problem made worse by federation.
You’d be surprised how hard the task is. Any given identifier can be trivially changed. IP addresses aren’t permanent and change frequently for home users. Multiple people genuinely use the same IP. People are likely to have very similar writing styles to someone else by complete coincidence. Browser fingerprinting can be thrown off by simply using another browser or fucking with the User Agent.
That about puts paid to any identifiers either Reddit or an instance owner may have.