Remove excess pages for higher rankings

This would be an excellent strategy for any proactive admin. Of course, there are more advanced ways to approach it as John has done. But simply deleting nonsense threads that have very little value is a great way to do it.

Why not just move them to an archive section that only members can see?

That's a reasonable SEO strategy, although you might just want to clean up any low quality threads anyway. It depends alot on the niche. Chat type forums likely end up with the most low-quality threads. The more specific the niche, the more likely threads are to be quality (I surmise).

I have always wondered, and even asked in various places, about making the off-topic area of my forums only available to members, to prevent the indexing of completely irrelevant threads.

After looking at my threads, the off-topic threads are the ones, for the most part, that get the least views. I imagine that the views that these threads do get are from the regular members who like to hang out in there and waste time.

Yes - if off-topic is really off-topic, then I think blocking it is a great idea.

I have decided to block my off-topic area on one of my forums, will probably end up doing the same for all.

I have been blocking all outbound links because of these forums and the tendency of members to post loads of links to outlandish stuff that has nothing to do with my site.

Also doing as John Scott is doing, and moving old non-performing posts. I am not moving them out of view of members though, just making archive forums for these threads as sub-forums. Not sure if this is related, but I did this a couple days ago, and googlebot has been checking my site out heavily for the past two days now.

One interesting method (which would require automation) would be to remove guest access to threads that are not *traffic bearing* within a set period.

Example: A page does not generate any clickthroughs, from any search engine, for 6 months. It would be deleted or made visible to members only.

This would help to push link consensus through to pages demonstrating the capability to pull traffic, and take it away from those that do not (due to poor thread titling, too high of keyword competition for the title phrases, etc)

You would need to track link backs and SE referrals for every single thread.

Yep.

Maybe an easier way would be for all threads to be nofollow and an admin or mod can click a button to remove that restriction if a particular thread is of good quality. It could at the same time allow it to be included in the site map. Then those that run the forum would have complete control over their cached pages.

That's definitely a very interesting approach. We don't want to be too discriminate though, to ensure that we maximize the possibility for long tail search queries that could occur.

Perhaps it could be turned on or off for each forum. Then you could click to allow it to be followed or click to deny. So a topical forum could be left default and noisy threads could be turned off. A non-topical forum could be changed to nofollow and the occasional interesting thread could be changed to give access to spiders.