It's very important to understand the difference between deleting and unsubscribing a contact because you must ensure you do not send to contacts who may have opted out from receiving messages from you.
Deleting a contact removes them from your address book; however, if they are created as a contact again you may inadvertently send them another message. Learn more about deleting contacts here.
Unsubscribing a contact means that as long as the recipient stays unsubscribed they will not receive a message from your account regardless of you including them in future broadcasts. Learn more about unsubscribing and re-subscribing contacts here.