Thanks for reaching out, your question about how to close your account and get a refund would be best suited for the support group at Mailchimp to review. Here at the Mailchimp for WooCommerce team, we focus on the engineering of the integration between your store and a Mailchimp audience.
They would be able to access your account and walk you through that process. You can reach out to our excellent customer service team here: https://mailchimp.com/contact/. You should be able to chat with them and take things from there.
