Cancel a contact's scheduled messages

Cancels every queued (not yet sent) message for one contact in a campaign, across all of that contact’s conversations. Other contacts on the same sender number are not affected, and the campaign keeps sending. Cancelled messages are removed and never sent. Messages that are already sending are left alone and not counted. Safe to repeat: when nothing is queued the response is {"cancelled": 0}. Returns 400 if the campaign does not exist on your account.