Your customers can unsubscribe themselves from your email list by clicking the "Unsubscribe" link at the bottom of each email you send. This will place them in the unsubscribe filter and prevent them from receiving future emails.
If your customer wishes to be removed from your list but will not click the "Unsubscribe" button at the bottom of your email, you can email us at email@example.com telling us your account name and the email address that needs to be moved to the unsubscribe filter.
You can also "unsubscribe" the customer by simply deleting them from your list. However, if they are entered back into your account either by spreadsheet upload or one-at-a-time, they will not be filtered into the Unsubscribe category.