According to the terms of use, any email marketing should provide the recipient an option to opt out.
To do this, any recipient would need to click on the Unsubscribe link in the footer of your email. The email footer and unsubscribe link cannot be removed and is compulsory to have for every email blast.
Once your customer/recipient/buyer/potential buyer unsubscribes, the subscription in the audience group will be updated from Subscribed to Unsubscribed.
You will no longer be able to send any marketing to this person.