When composing your SMS campaign, you will have the option to include a URL for your contacts to opt-out of communications.

To add this URL, click on the 'Add unsubscribe link' button

Keep in mind that Instasent generates the URL, but you are responsible for composing the text that will be displayed (such as "Stop" "Unsubscribe," etc.)

The {{unsubscribe}} value will be replaced with the unsubscribe URL.

A user whoe unsubscribes will be named Unsubscribed user, and their 'Subscribed to SMS' attribute will be updated to "NO" in your audience.

Prior to confirming a campaign, you will find this option:

It is highly recommended to always keep this option checked, as it will exclude those contacts from the selected audience.

You may also be interested in knowing the difference between the SMS acceptance attribute and the SMS subscription attribute.