Well I wonder why no one has replied with the obvious candidate, the Press section. Most of the spammers there were banned but some still seem to post there for sig, repeatedly but just after a few other posts in other section to not seem as a spammer.

Another recommendation is Local board's offtopic section. While you're at it, might think about Politics and society and speculation. Seems like the new breeding grounds for "constructive"/Off-topic sig spammers.

Recommendations that I agree- Off-topic, Games and rounds(obvious choice) and Auction

And well digital goods do have constructive posts, they should not be "deleted"/not contribute to post count for the sake of a few spammers.

Edit: Also, don't the auto-sig bots take the posts from "Last posts" and discard the posts in non-paid sections? Pretty sure Bitmixer and Yobit won't change the script as their signature will still be displayed. Although this will stop a lot of potential activity farmers from making useless posts