This article will outline how to archive/cancel a pending or repeating marketing campaign from sending once it has been scheduled.
Go to Marketing > Campaign History and use the filters at the top of the screen to find the scheduled message.
A repeat/recurring message will be listed as Repeating.
And a scheduled message that hasn't been sent yet, but is scheduled to send at some point in the future, will be listed as Pending.
To stop the message from sending, single-click it then click the Archive button.