I really don’t understand why people want to disable comments on old posts. I see people wanting to do this in the WordPress forums and on various blogs (and in comments). There is even a WordPress plugin for this.
This seems like a better approach to me. Comment spam is like e-mail spam, any 
solution 
A good idea that doesn’t hold up
Speaking of comment spam: Alex advocates not shutting off the comments on a blog but instead running them through a moderation system, maybe even one that looks for probably spam words. It sounds good, and it’s true you won’t get any spam becuase onl…
If you still had to manually deal with each spam, your toolset was insufficient.
I have only had a few comment spams so far, so maybe I don’t yet realize what a major problem this could be. Up and until now I’ve just gone and deleted the particular comment spams that were a problem.
Alex: People want this because they don’t want to be bothered. Many Webloggers are fundamentally very lazy, and even the minimal amount to use comment moderation is a bother to them.
I didn’t want to say this in the comments where Todd mentioned it, obviously, because, well, we’re friends, and he’s still way bigger than I am.
I want to turn off ALL the comments because I don’t have time to deal with right now and probably won’t for a few weeks. In the mean time I don’t feel like deleting bunches of comment-spam everyday. I just want it to go away until I have time to deal with it.
It would be really nice if this were an option in WordPress but I’m gathering that it’s not so I’ll just have to keep deleting it all until I time to fix it.
Try posting in the WordPress support forums.