It's very important to understand the difference between deleting and unsubscribing a contact because you must ensure you don't send to contacts who may have opted out of 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 won't receive a message from your account regardless of you including them in future broadcasts. Learn more about unsubscribing and re-subscribing contacts here.