Turning on captchas didn't even help, because these spams are apparently all entered by actual humans, who are making an effort to appear to be on topic if you don't read the URLs.
At this point, with the useless lack of tools provided by Livejournal to address this problem, my only choices are to disallow commenting from non-friends (in which case I might as well stop using LJ) or moderate comments from non-friends (which is even more work than deleting spam).
The only thing that has so far prevented me from giving up on LJ and hosting this blog myself is that I think I would get less comments. Maybe that's not even true any more, with all the tumbleweeds blowing through this site.
Mostly the spam happens on posts that are months old, presumably because they have more google juice. I'd be ok with just turning off comments on old posts -- except that when you turn off comments on an LJ post, all the existing comments disappear.
I think the first time I submitted a support request to LJ to make it possible to turn off new comments without hiding old ones was in 2003. Apparently adding new "valentine's day gift" logos has remained a higher priority than spam fighting.
So if you know anybody who still works at Livejournal, please stab them in the face.