Comment spam seems to be resurgent at present, disappointingly since the nofollow tag ought to make it pointless. I deleted a whole bunch of the usual stuff today, at least the fraction that got through the Textdrive outer defences and my moderation rules. But there were a few I couldn’t make much sense of, like this one
I can’t work out what gives here. The spurious poster links back to msn.com, presumably not the source of this spam. Can anyone explain this to me.
John Quiggin 
Sometimes spammers will test the limits of your spam filters by posting seemingly random comments without an actual link payload. If they include sites (often yahoo.com or, in the example you pointed to, msn.com), it might be to poison your blacklist of URLs, so that legitimate comments begin to be trapped — they hope you’ll slacken your defences. Once they’ve established that their comments are getting through, they will come back with a wave of real spam.
Beats me. But hey, I’m just here for the odds. Roll ’em bones, Cap’n. Baby needs a new pair of shoes.
I think I’ve mentioned before that the “no point” argument about spammers doesn’t work, any more than making it pointless for rhinoceros poachers to kill their prey by removing their horns. The cost of a wasted hunt is so much that poachers shoot the beasts anyway, since the cost of a bullet saves them days of wasted tracking later.
Similarly, it’s quite cheap to shoot a spam at you on the off chance, once they’ve found your site. Most of their costs are sunk by then, and checking your blocker’s limits indirectly costs more than just trying a spam and seeing what happens. Your method would only work if enough bloggers did it long enough that no spammers bothered with any blog spam any more.
We are under attack by a Mexican. I wonder if your spam has the same source.
Not a Mexican Hamster, by any chance?