A pay-to-post subforum would be an excellent improvement, for those whose time is valuable and just want to focus on the business-oriented (and domain name!) threads. The sub-forum could be read by anyone, but for posting privileges, one would pay $$$ (either per post, or for a flat rate per month). I believe the newer versions of vBulletin support Paypal and other payment systems, for different classes of member subscriptions, too, so it wouldn't be difficult to add.
As an experiment, maybe make it a $5 flat rate for a trial month of March 2005, and then adjust it depending on what happens?
One could do the same for signatures, $5 to be able to have one.
That would cut down on the "sig placement" posts.