I had the same issue with classic at that time.
Even though threads is very different.
From memory:
What I did was register several users using a specific distribution email address for each.
Then logged on as each user and subscribed to the pertinent forums.
Then banned the user so stray emails would not be sent to them from the board and so they would not show up as users in the user list.
No file hacks were needed.
Then I added a list(island) to subscribe to on the main page.
Then in the list manager I added a footer to explain how to unsubscribe to the list.And a note not to reply to the email.
I also had to set security measures in the mail list for only subscribed members to limit spam. Meaning the main board email address was allowed to post.
It worked very well at the time.
