In a nutshell: i imported a couple mailboxes to a library, then thought better of it, so apple-Zed them away. However, the forwarded and replied tags remained, and will not go away. When selected, the delete button is ghosted. I made sure there was no mail in the trash, or anything.
The “forwarded” and “replied” tags are part of a small set of built-in tags, which cannot be deleted. I plan to add more controls so that you could, e.g., hide the tags that you don’t want to see in the source list.
EagleFiler 1.1 added support for hierarchical tags, so you can group tags that you don’t want to see under a parent tag and then twist it closed.