Over the weekend I attended a WordPress class at which I learned several interesting tips and tricks. Turns out it is fairly simple to just block a website (any website, not just a WordPress site) from visitors based on IP address. So, I found a site that publishes various IP address lists, and picked out one that had the known US-based IP addresses, and I set this site to deny any not on that list. For this site, that is a reasonable thing to do, since my target demographic is residents of north central Texas.
I did that a couple of days ago. As a result, I saw my traffic drop by about 40%, which is not a bad thing since it means that the traffic I am now getting is much more highly focused on what I have to offer. Plus, the traffic is building up again.
The spambot comment traffic has slowed to an easily managed trickle.
Unfortunately, the IP address restriction caused another, subtle, problem. I have removed the restriction, and I will either continue to handle the spam problem manually or find another solution.