I have installed the MT plug-in MT-close, which will close comments threads after 7 days. This is an attempt to ward off spam, since all of the content spam I have had to clean out has been from threads that have been cached by Google, and are more than a week old.
I haven't had much in the way of relevant comments more than a week past the date they are posted; the only ones I have received are from people who self-google their names, and find them on my pages (usually from a "letters to the editor" citation). Since they can still e-mail me if they are outraged, they are free to do so.