Good news, I was able to petition for control of the Nabble forum. Turns out, no one in our group had any control over the forum, it was only Nabble employees.
Anywho, I have added the same group of admin from github to nabble (Lieven, Marc, Michael, Jim. If you don't have a nabble account, when you sign up you should have admin access).
Now, here is what I propose.
1. Lock down the Issues and Announce sub-forums so that no one can post there, but they are still searchable.
2. Add a banner at the top of those forums explaining that they are read only.
3. (Most controversial) Possibly lock down the main forum as well. I believe that this will prevent nabble users from directly posting to the forum, but the forum will still display messages sent to the Sourceforge mailing list. Essentially, this would make the nabble forum a read-only repository of messages. I believe that currently, if a nabble user posts directly to the forum, without using the sourceforge mailing list, the messages appear in the nabble forum only and are not sent to the mailing list.
3. (Alternative) I have no idea if this is possible, but currently when you post to the main forum at nabble you get the following message:
```The mailing list may require your subscription before accepting your post. Please note that being registered with Nabble does NOT automatically subscribe you to this mailing list. If you haven't subscribed yet, please do it now. If you aren't sure or don't remember, just subscribe again because there is no harm.```
Does anyone know if there is a way to alter the sourceforge list so that it will accept any message posted to nabble, even by an user not registered on the mailing list?