I know the mod tools on Lemmy aren’t terribly robust, but are other instances able to block individual communities without defederaton?

Actually- that is honestly a good question

While- I can block a community, I do believe that may only affect my account. The purge/remove options exist as well, but, I do believe those are also temporary.

That being said-

At the database level, community_block is defined for a particular person.

Looking at the removed option,

From an admin's perspective, it flags the community as removed.

But, from a user's perspective- the community no longer exists.

That being said- I do believe the remove option might be suitable for this purpose.