After having some issues with spam comments over on his blog, Ryan Malesevich decided to do something about it. So, being the WordPress fan that he is, he implemented a new plugin to help things a bit - removing the ability to comment on a post older than 21 days.
I'm using a plugin called Auto-Close Comments. Whenever you write a post, edit a post, or anything similar to that, it will go through the database and close any comments for posts older than three weeks. I'm going to try it out for a while to see how my spam comments are altered.
You need to install it manually since there's no official packaged plugin for it, but it's a pretty simple matter of cut & paste to get it working. Then, you just define the number of days that you want the cutoff to be and you're all set.