Yes, we can automatically unsubscribe a contact based on a keyword being at the start of a message.
The keywords that are currently in use are as follows:
The keyword "Stop" is generally used as it takes up fewer characters in an outbound call to action.
A typical example of an opt-out message (inserted at the end of the message content) would be "Reply Stop to opt-out"
Note: The recipient must respond with the opt-out keyword at the beginning of their message i.e. "Please stop" will not work and this recipient should be manually unsubscribed (see How do I manually unsubscribe a contact )