BuddyPress.org

I went into the group activity page template file and just surrounded the update textarea with conditional tags, and hid the dropdown “post in” list on the sitewide activity page altogether (left the option to post to user’s profile selected and the whole dropdown hidden).

Now group members can post replies to admin/mod group activity stream updates, as well as do everything they used to in the forum, but they just can’t post new updates to the group activity stream.