When a customer first receives a text from your brand number (either as a confirmation message or as an opt-in message) it will contain the instructions on how to opt out - at any point they can reply with "stop" to cease all future communication.
As soon as a message is received into Blueprint containing just the word "stop" (or variations such as 'STOP' or 'Stop'), our messaging service will automatically block that number and their number will be removed from the dashboard. Please note that it can take a short period of time before they are fully removed from the dashboard. If another message is sent to that customer before they are removed, the message will not be delivered and marked as 'failed' (this will also not count towards your monthly message allowance).
Once a customer has replied with the word "stop", you will no longer be able to reply to that customer and the text box will turn grey.
To manually opt out a customer, please read this article.