Yargh. The mess in dealing with comment spammers is worse than I could have imagined. Apparently they are accessing the "mt-comments.cgi" script directly, which means simply turning on comment moderation is not going to cut it. I've posted a plea on the MT forums in hopes that somebody can tell me how to make it so the script rejects all attempts to comment when the request doesn't come from my "official" comments form... but I don't even know if that's possible. If there's no way of rejecting outside direct-access to the script, then I guess comments will have to be turned off permanently.
That's kind of sad. I like getting the occasional comment from time to time.
Even worse, now that I'm had to turn off comments on all entries more than 14 days old, there's no way to tell people that comments have been closed which is very confusing. I thought a little MT template wizardry would fix this but, alas, my attempt has failed...
UPDATE: Many thanks to Neil and James who have pointed out that the "MTElse" tag needs to be within the conditional tag in order to function properly. Once I moved the block to sit within the "MTEntryIfCommentsOpen" set, everything works perfectly!
Comment spammers suck ass. The internet used to be fun before evil people had to go and ruin it for everybody.