If you have a scheduled mail merge campaign, it will show under the GMass Scheduled Label. To cancel the campaign, find the campaign under the GMass Scheduled Label, open the Draft, click the GMass Settings arrow, and hit the Cancel button.
This also applies to a large campaign that has been set to send over multiple days. If after a couple of days, you decide you don’t want the rest of it to send, follow the same procedure to prevent further sending.
Important Note: If you have auto follow-up stages assigned to a campaign, then canceling the campaign after it has partially sent will NOT prevent auto follow-ups from going out for people that have already received the email. To also cancel auto follow-ups, set each Auto Follow-up Stage to 0 Days, and hit the GMass button to save it.