PSA: You can now lock comments on old LJ posts.
It turns out, however, that you can now do that. (I think it's a relatively recent addition that nobody noticed because it didn't tick off half of the userbase.) Edit the entry, and change the "Allow Comments" setting to "Locked". This is documented as the second bullet point under "Changing Comment Settings on this FAQ entry.
Dreamwidth doesn't support this yet, but I'm going to go file a feature request as soon as I post this. I'm not sure what happens if you set comments on a LiveJournal post to "locked" and then import the post into Dreamwidth.
Crossposted from Dreamwidth (original here), with comments. Comment here or there.