MT-Workbench, a PHP class library that can close all weblog entries to comments and trackback after they become one week old.
The open source library was written for a Movable Type weblog that uses a MySQL database. The library's closefeedback.php script can be run daily as a cronjob, closing old discussions so they can't be abused by comment spammers trying to boost their Google PageRank.
So far, the results have been heartening on the Drudge Retort, which used to receive 5-10 spams for every 100 visitor comments. In several skims this week, I haven't been able to find any spam.