In my interaction with the Internet and spam, I have learned a few odds and ends.
I suspect that the person sending the messages (usually through an automated system of some sort) and the person advertising are usually separate, and probably only meet through electronic communication methods.
I have seen claims (I have no idea of their validity) that large amounts of email spam come from countries (like Spain or China) where people out for some quick money will find every possible way of delivering advertising to email, comment-lists, newsgroups, etc.
I do know that spammers tend to attract certain kinds of business--business of the type that I categorize as "tabloid". Theis business is advertised towards the kind of people who purchase tabloid news in the supermarket.
Oh, well. That's my mental model for how these things work. My best guess is that the spam-senders will do anything to drop a web-link to the advertiser, and bill the advertiser per link that they create.
Or they are pranksters, who do this for fun. Which makes slightly less sense than writing email-viruses.
You sound like a candidate for MT Blacklist. We have it over at notfrisco. Here's their theory of what's going on: "Unlike email spam, the motivation is not to sell you or your visitors anything or even to get you to click on their links. The most solid theory for why comment spamming exists — and what makes the battle against it very different from the previous scourges — is that by placing links all over the web, spammers increase the Google PageRank for the sites they are hawking.
In order for this scheme to work, the links must be published on as many sites as possible for as long of a time as possible, but at least long enough for the Googlebot to see them. This is where MT-Blacklist's Search and De-spam mode comes into play. MT-Blacklist makes it terribly easy to recover very quickly from even the most intense spam attack with almost no effort, rewarding the spammer with no extra Google juice. Furthermore, for efficiency reasons, spammers tend to put twenty to thirty URLs in each comment but it takes only one known URL to block it."
Let me second Camassia's suggestion of MT-Blacklist.
I had a deluge of comment spam at the first of this week (about 20 a day) and finally upgraded to MT 2.6 and installed the MT-Blacklist plugin. It was simple and it is effective. Zero comment spams since then.
Since you alreadyuse MT 2.64 it might take you 15 minutes to end your problem.
Thanks for the suggestion. The spam isn't too much of a problem -- after taking a few days away from the blog, there were all of two posts.