The proper way would be to probably make an API client that logs in to each of these accounts, queries the list of subscribed communities, and unfollows each community. But I’m not even sure the backend deals with all this correctly… I’m working on additional testing to validate the backend behavior.
I don’t really want to spend resources on fetching new posts and updates.
As you have discovered, your instance is not initiating a “fetch”, it is actually the other servers sending to you. Those other instances have your now-gone users (on your server) still subscribed to the communities.
Short of defederating, you have to get the other servers to remove your subscribers to those communities, or at least unsubscribe. This kind of thing is currently not trivial to do, likely requires new code or direct SQL statements for those other servers to execute.