I'm still subscribed to this topic though no longer active here (see 2) below).
A few thoughts- let's say there are 1,000 new posts each day*, and the board has an 80% spam rate- in other words, 800 of the 1,000 are spam so there are 200 approved new posts per day. The ratio would be the same if there were 100,000 or 100 new posts.
1) Since all "new" posts are being moderated, is it consuming more time to read and (approve or delete) 1000 posts or delete 800? In my experience, giving members the ability to report spam in new posts makes the moderator's job a lot easier (I see this forum allows the report post function). Kind of like neighborhood watch.
2) If you make the path to do something too difficult, humans like water & electricity will take the path of least resistance. In my case, it was utilizing a different product when I couldn't get questions answered or information in a timely manner. Due to other projects and activity, I have a set amount of time I can devote towards a specific forum for a specific project.
3) After a few posts, new members should be able to be "profiled" by staff to determine if they are in fact real people posting relevant info or the aforementioned spammers. If the former, they could then be given full posting privileges.
4) An IP ban for offending addresses can be written using most forum soft ware (this word split to avoid an ad link ). The 23,000 member automotive board I moderate uses SMF, I see this one uses VBulleitn. It's an admin and about 4 active moderators. I spent about a week writing IP bans a year ago and to date we've only had a couple of legit users fall into this to where they couldn't post. It's reduced our spam by nearly 100%. The few that get through are reported within 24 hours and the user account and/or IP address handled.
*VBulletin does give some stats. As of right now:
Currently Active Users
There are currently 2413 users online. 56 members and 2357 guests
Most users ever online was 4,401, 08-10-2011 at 03:54 PM.
7,438 new posts since my last visit on 7/14, which was 80 days ago- 7,438/80=avg. of 93 posts approved per day.
No hard feelings, just sharing a point of view & some info.