You can cancel scheduled mail merges and follow ups through the Mail Merge interface.
If you need to cancel an entire scheduled mail merge or cancel stages, click into the campaign and select the "Stop Mail Merge" button in the top right. Canceling the mail merge will prevent any scheduled stages from being sent. Once it has been canceled, the mail merge will remain in your campaign list but with a "Stopped" status.
You cannot cancel mail merges which are in the process of sending.