Akismet is the best comment spam protection plugin which comes by default with WordPress installation. Akismet provides the basic protection to fight with web spam. By default Akismet comment settings automatically delete the comments after 30 days from its database which it has considered as spam.
However you might not always like this default settings specially when you need to track the spam comments later or need to know the number of spam comments you are receiving each month.
How to Configure Akismet Comment Spam
If you are using Akismet for the first time, then you need a API key, which you can find from Akismet website upon registration. The API key can be used on unlimited WordPress sites that you own. Once you receive the API key, login to your WordPress Dashboard
Go to Plugins > Akismet configuration
Enter the API key and click “update options”. Akismet will verify the API key and activated immediately upon verification.
Now to prevent Akismet to auto delete spam comments, untick the check box –
Auto-delete spam submitted on posts more than a month old
Save the settings and Akismet comment settings will now stop auto deleting spam comments which has passed one month and so on.
If you have somehow deleted Akismet Comment Spam plugin, you can always download it from WordPress default repository.