Thanks for your reply.
Yes, Akismet's catching the spams.
The post doesn't have any legit comments, so I could turn off comments on that post, though I'd hoped for a less "brutal" solution.
There are a couple of pingbacks from (legit-looking) Brazilian sites on the post, so I'm now assuming those sites might get a lot of spam, and the bots just follow the links from their pages to mine.
I tried deleting the pingbacks from my post, but it didn't make any difference -- which in hindsight makes sense because it's the incoming links that would be a problem, not the return links from my post.
So if the incoming links are the problem, it sounds like your idea of banning comments on that post might be the most effective (or maybe the only!) change I could make.