Anyone can automatically opt out of receiving your texts by simply replying "STOP" to any of your messages.

This will automatically take their phone number out of your subscriber list.

It will also put them on your account Opt-Out Report, so you'll be able to view who all has opted out of your texts:

People who are opted won't be able to receive your messages unless they text "UNSTOP" back into your account number. If UNSTOP does not work, have the subscriber try “RESUME” or “START”.