There's no perfect solution, but at least you "save" the business section.
I've not noticed the spam, the moderators must be doing a good job
Defining a new user as someone with less than 10 posts, I would attack the problem with the following steps, in the order specified:
1 - Define a "new user" concept as someone with less than a specified number of posts and apply a time interval restriction for new users (to prevent spamming).
2 - revamp the registration process:
i) activation link sent in email (and ban some email domains like "maillinator.com")
ii) time interval restriction on registration from the same IP (to prevent multiple registrations on the same computer)
iii) temporary IP probation (not very effective)
3 - implement functionality to assist the moderators job:
i) hide all posts from one user (avaiable for new users)
ii) mute a user
iii) delete the user and all it's posts (avaiable for new users)
4 - Initial probation period in wich the posts would have to be allowed by a moderator
Only step 4 is unfriendly, but if necessary, I don't see another way.