You should always provide a simple way for your subscribers to opt out of receiving messages if they don't want to get your campaigns anymore.
If you are sending emails, you will have an unsubscribe link at the top of the email.
If you are sending text messages, your subscribers can text "STOP" to opt out.
If you are sending voice broadcasts, you should allow your subscribers to press a certain number in order to opt out.
Providing a simple opt out method keeps customer satisfaction high so customers don't get frustrated or report your messages as spam.