Comments are really important to make a community around a blog but they are the most used source of spamming as well. Spammers used to run bots to populate your comments section with lot many links related to some sites. Yes, Akismet is there to filter them as Spam but still those comments occupies nice amount of space in your database (if not deleted). If you are having a popular blog then you must be getting hundreds (even thousands) of spam on daily basis. To make your life bit easier, you may use the option to close comments on older posts.
Most of the time people used to comment on latest posts or newer posts. In WordPress Admin panel, you may choose the option “Automatically close comments on articles older than some days”, you can set the number of days in the box provided over there. Here is the location of this option on WordPress Admin Panel,
Login to WordPress Panel ==> Settings ==> Discussion ==> Other Comment Settings
Here you can see an option as “Automatically close comments on articles older than some days”. You can change the limit (in days) by providing the value in that box. Most used value is 90 which means no one can comment on a post older than 90 days, which is nice amount of time to comment on a post.
DISQUS users can also set this limit thru Disqus admin panel. To do that, go to the Settings tab in your DISQUS panel and then change the limit in “Moderation” panel to some value so that comment section will not appear in the posts published before those many days.
To avoid Spam on your blog, you may use Mathematical Calculation Plugin which will ask the visitor to enter the correct sum of a particular set of numbers. But in case you are using WP Super Cache on your blog for caching, then you may face a bug with that plugin. Visitor may not be able to drop comment because this plugin will throw error even when the visitor is providing correct answer. You will have to delete cache in that case.
So you can trust Akismet and set the number of days for no comments to minimize the number of spam per day.
Enjoy the spam free blogging.