The following tutorial removes your accounts from the database, this includes the account and the lists only. However stats and designed opt-ins will remain.
Though it's always a good idea to back up your website first.
Go to Dashboard > Bloom > Email Accounts, and click on the trash icon to delete the account (with the lists):
Divi Email Module
Simply open the module settings and click this button:
If the standard way isn't able to remove the accounts for some reason, you can remove it with a PHP code.
Note: This method will remove all accounts and lists. Please back up the site before you try this.
Open the header.php file of your theme. You can use the editor from Dashboard > Appearance > Theme Editor.
Search for this line:
<body <?php body_class(); ?>>
And add the following code after it:
<?php update_option( 'et_core_api_email_options', '' ); ?>
Open the front end of your site just once (or refresh any page on the front end). The accounts should be removed now.
Important: Remove the code after that. Otherwise your accounts will be removed each time someone visited your site.