Firstly, there is no setting for not deleting spam comments. My confusion was caused by ambiguous wording on the Akismet configuration page...
Auto-delete spam submitted on posts more than a month old.
"more than a month old" is referring to "posts" that are more than a month old, not "spam submitted on posts" that are more than a month old.
As far as the original question about how to stop the comments from being deleted...
They are automatically deleted after 15 days, no matter what. Akismet thinks they know best! They assume you want the spam deleted after 2 weeks because they also assume that 15 days is more than enough time for you to verify that the comment was flagged correctly.
I asked them why they couldn't simply give the user an option and I got some excuse about how thousands of spam comments can shut down a server. Fine, that's true. But the same thing could happen if the Akismet plugin wasn't installed in the first place. Give the admin a little credit for being able to make his own decision regarding his installation. But that's not going to happen.
I realize editing core files is frowned upon, but here's how I decided to handle it for my own site.
This line in the
$comment_ids = $wpdb->get_col("SELECT comment_id FROM $wpdb->comments WHERE
DATE_SUB('$now_gmt', INTERVAL 15 DAY) > comment_date_gmt AND
comment_approved = 'spam'");
INTERVAL 15 DAY to 180 days, giving me more than enough time to review comments before deletion.
Every time the plugin is updated, such a modification would have to be re-applied.
As per helpful suggestion by @ChipBennett, I created a plugin which does nothing but deactivate the comment deletion by Akismet plugin. This simple thing avoids the whole issue of having to modify the Akismet plugin.
I simply created the following
php file and put it in the plugins directory (
* @package Akismet_keep_comment
* @version 1.0
Plugin Name: Akismet Keep Comment
Description: This plugin removes the comment deletion ability of the Akismet plugin
Author URI: http://www.mysite.com
Then I "activated" it via the plugin page of the Dashboard.