A site I administer started getting ridiculous amounts of spam after reaching a certain level of traffic...and not just the spambot type; like sweatshop staff going around signing up just to post stuff.

Wound up developing a multiple layer system to ensure every user was legit, including a human check (answer X question), admin/mod approval of new accounts (including a whois check and comparison to existing user known IPs), and then all new users' first couple posts are quarantined pending mod review.

Maybe overkill for DN, but it had worked for me..Haven't had a single spam post in years.