When a person has requested to opt out of receiving future messages, you will see the opt out message in the inbox of your account.
You should select the box next to the message and click the Opt Out button. You can also select multiple messages to opt out multiple requests.
You are also able to search for a contact in the Contacts tab of your account and opt out directly.
If you have gathered a list of numbers that no longer wish to receive messages, you can copy and paste them into the Mass Unsubscribe box at the bottom of the page in the Upload Contacts section of your account.