I just installed a very handy wordpress plugin on a couple of blogs.

Comment spam is obviously a very big issue in the blogosphere, and while there are some fantastic anti-spam plugins out there, they don’t catch all of them. A good way to solve a lot of this is to close old posts. There isn’t really many occasions where comment discussion continues after a couple of weeks, so close all new comments. The majority of blog comment spam is done on your older posts in the archive.

This plugin will automatically close your comments after  length of time you set (the default is 14 days). Old comments can still be read, however a user cannot post a new one.