When history was created today with more than 300 spam comments on my blog, I was at the beginning stages of madness. But before I went over the edge, I discovered this beautiful MT plugin. This plugin closes comments on entries which are older than a user-specified number of days.
What’s exciting is that it is just a cgi page that can be copied to your base MT installation. Just make sure you set the read and execute permissions for all groups of users.