Thanks for writing in! It can be! Whether the Preferred Format option appears on your form is dependent on your list settings, inside of MailChimp.
To disable this, login to your MailChimp account and go to the Lists tab. Navigate to the list you’ve connected your plugin to and click on the gear/cog icon on the right-hand side, for that list. Then, select List Name & Defaults. From this page, you should see the option to remove the Preferred Format option from your list’s sign up form.
After you’ve disabled it inside of your MailChimp account, then go back to your WordPress Admin area and go to Plugins > Installed Plugins > MailChimp > Settings. From there, update your list to refresh the new settings.