We've got a handle (mostly) on the spammers now. They're still here 2-3 times per day, but we haven't had any get out of moderation for a couple of weeks now. We're still looking for ways to stop them completely, but until then, we'll keep the current "processes" in place:
Forum accounts that have a serial number associated to a current subscription are never moderated (unless you add an external link to your message).
Forum members with 6,000 points or more are not moderated (unless you add an external link to your message).
Forum members with less than 6,000 points will be moderated initially, and will be added to a "vetted" group after a message or two (to make sure you're real). This means that some long-time forum members may suddenly see their posts moderated at least once. So far almost 200 members have been "vetted".
Thanks to everyone who helped by flagging posts, sending me emails, text messages, and/or Twitter DM's. I hope I never have to receive them again