True, it "could" technically be a plugin. But, then again, it would let spammers know it exists.
Me personally, I think the "best" option long term is to filer any posts on a site less than say, 2 weeks old, through a filter(s) like SK2 uses.
I looked at SK2, to see if it could be hooked into easily, but it's centered around comment_id and comments very heavily. It would require hooking into the actual plugin filters SK2 uses, or creation/modification of existing filters to pull off. It would eat enough time that I had to push it down my list a good ways.
You also run the risk of some false positives, like someone blogging about their weekly poker night or whatever, but I'd rather have a 5% false positive rate and 0 splogs, than 5% splogs and 0 false positives.
You know, I should make a "bulk email" address, and since these chumps are forced to use a legitimate address to register, start sending them an "invoice" for my time. Not that they would pay it, or even read it, but just for spite.
Either that, or start publicizing that address and registering it for "a ton" of newsletters, so that their mailbox is filled up and they can't use it.
Or maybe when they try to log into the site again, redirect them to an alternate site. Spamcops, or something else that's annoying. (I'd list em, but let's just say they "aren't work safe", if you get the drift.)