It is an "industry standard" and requirement when using the short code to always give a way for the recipient to Opt out of receiving messages from you. Because of this, some of your recipients might have opted out of receiving ALL messages from you by replying with the word STOP when they only meant to opt out of that particular text list.
It is required to have the exact words "STOP to Opt Out" in your signature when using the short code, but you can add words to that signature to make it more clear or to give a different opt out word to use that won't unsubscribe them from ALL future communications from you.
Here is a video overview of this from one of our Tip of The Weeks:
1. Click your name in the top right corner of the screen and go to your Organization Settings page (only owners have this option).
2. Edit both signatures - the reprocessing signature (A) and the one used when sending group messages (B).
3. Change of the wording to give a different reply if they want to be removed from this particular text list, and clarify what replying with STOP will actually do (unsubscribe them from ALL future communications).
Ex: "~ From [Org Short Name]. Reply REMOVE to be removed from this list, or STOP to opt out of ALL future messages from us."
4. Once you have made your changes, click "update" to save.
When someone replies with STOP or Unsubscribe, their carrier handles the opt out. The only way for them to re-subscribe is by replying with the word START.
When someone replies with REMOVE or anything else you prompt them to reply with (REMOVE ME, TAKE ME OFF, etc) you have the ability to manually remove them from the group / text list so they no longer receive messages from you for that list. They will be able to receive other messages still.