When mobile subscribers send a "STOP" message to your Plivo number, they're considered opted out. Plivo will automatically block any subsequent SMS messages sent to these phone numbers. If you attempt to send messages to an opted-out customer, they'll fail and return error code 200.
After receiving a "STOP" keyword from a customer, you can acknowledge their opt-out status by sending a single SMS within five minutes of receiving the keyword. However, this is the only message you can send to opted-out customers.
Plivo provides a default "Do Not Disturb" (DND) feature for long codes and toll-free numbers in the US and Canada, which can also be extended to short codes. To set up DND for short codes, please reach out to Plivo support.