Yes. You can cancel the generation of any merge at any time. The "Generate" button turns into a "Cancel" button. Simply click on this Cancel button, as shown:
A Summary Report indicating the line where the campaign was cancelled would be generated:
If you schedule a future-dated merge, such as "In an hour" or "Every hour" or something that repeats on a schedule, the "Generate" button turns into an "Unschedule" button.
Simply click on the "Unschedule" button to stop your merge.
You should see a confirmation message that says "The campaign has been unscheduled", and the "Unschedule" button will turn back into a "Generate" button.