Had some other thoughts too, not sure if the forum software supports this kind of configuration though. But here it goes ...
The problem statement is too many repetitive threads (#1), but actual problem is difficulty in browsing Forum and find information (#2). We should attempt to address both, as they are related.
I feel that #1 can be best addressed by a manual process (what's being done now). Yes, there will be more work, but that can be addressed by having more category-wise admins. Only the process can be made clearer, something like:
1. Members advised to not reply to repetitive Threads
2. A button for members to flag/report Threads as Repetitive
3. Members issues Warnings by Mods for threads found repetitive, along with links to at least one existing Thread/Post on same topic. Each Warnings automatically revoked 1 week/month after being issued
4. Mods delete full thread so it no longer appears in Forum
5. Members who cross a certain number of warnings (3) get Banned for some duration.
Of course, the above doesn't address repetitive questions being asked on existing threads. But I think that follows the natural course of a conversation and can be addressed in conversation itself. I think we can & should live with it!
Now to problem #2 (easy navigation and information retrieval, weeding), some ideas...
a. I'm of the opinion that the "General Chitchat" section needs to be excluded somehow from "New Posts" search and "Unread posts since last visit", as it has too many un-categorized threads. But the section is quite a friendly area and I suggest it be allowed to live
b. Some way for Admins to mark individual Posts as Sticky & tag with keywords. Enhance Search feature to allow Tag search. This will help people find information easily before starting a new Thread
c. Some way for members to mark individual posts as Useful, so they go back and refer to it. Support for tagging in this feature also.
d. In each subforum, display Tag cloud that allows users to quickly view Sticky posts for popular Tags.
e. While creating new Thread, ask for at least 3 tags to be entered. System searches against tag cloud and suggests existing posts with similar tags. User can still go ahead and create the thread if they want.
f. Some way to mark threads as "Not Useful". Threads that are marked as Not Useful by at least 50/75/X people don't get shown in forum and search by default. But they can be searched for, using Advanced Search feature. This will help weed out threads and make sure they don't get in the way.
Sorry, it's a long Post, Thanks for your patience.
Cheers!