Dear fellow WordPress users, sooner or later you realize that spammers love old posts open for comments, but that there is no standard way to manage the comment and ping settings for -- say, 1600 -- old posts except by adjusting them one at a time.
But there is a plug-in, which I've added. Oh, happy day. Very easy and it seems to work. I've set all posts older than two months to no-comments, no-pings. That should keep the spammers away and Askimet -- my spam-culling plugin, with over 12,000 pieces of spam caught -- a bit more relaxed.
Extended Comment Options for WordPress (Being Mr. Kenny)