This section of the Server Setup | Email | Digest tab lets you control what users are signed up to receive the default server digests.
By default (unless you change the default), all users with an email address configured in Personal Information are added to the digest.
To remove users from the list, select the users you wish to remove and click the (- button. You can move users in the other direction using the +) button.
By removing the user from the list, you are making the same change that users can make in opting out of the server digest in the Account Settings / Subscriptions / Digest - Server Digest preference.