Future posts and comments will still come in. The remove and purge features don't tell the remote instance to stop pushing updates.

Remove is only hiding the community from non mods/admin. Posts and comments still get updated.

Purging is deleting all posts, comments and images. The community will be re-created quickly, because the remote instance is still pushing updates.

I even tried re-deploying my instance with a fresh install, but the "stuck" communities begin appearing immediately.