Removing Customers from Marketing Emails

Hi there,
I would like to send coupon codes via a marketing email on my opencart site: Version 2.1.0.1
I have managed to send an email via 'mail' in the admin panel - I sent it to a test customer group I set up and it arrived fine.
I would like to target customers based on which products they have bought though and would like to add to the bottom of the email a clause whereby customers can opt out of future emails. You can reply to the marketing emails, so we would know when a customer has replied opting out. However, I can't find a way to disable that particular customer's email, to take them off our email list.
Through the admin panel, I have tried to edit the customer profile and next to 'newsletter' switched the option to 'disabled'. However, this had no effect and my test group still received the emails.
Does anyone have any other ideas on how to remove customers from marketing emails?
Thanks!

To be honest, you're probably better off sending marketing emails like this from MailChimp. We've just developed an extension for a customer that allows them to select certain MailChimp "Groups" and "Interests" and which categories in OpenCart these relate to and then when someone buys a product from one of those categories they get added to the relevant group / interest and can be easily segmented and targeted for relevant emails.