Cancel scheduled messages

Cancels messages that are waiting to be sent in a campaign. Send {"all": true} to cancel every waiting message in the campaign, or {"messages": [...]} with up to 500 sessionId and messageId pairs (from List scheduled messages) to cancel specific ones. Cancelled messages are removed and never sent. Messages that were already sent are not counted. Returns 400 if the body is invalid or the campaign does not exist on your account.