A user can cancel sending while the activity is scheduled and while it is still in progress. This includes when a broadcast event is pending, while queue preparation is running, and while queue items/files/tasks are still being processed. The control is available in both desktop and mobile activity editors.

The Cancel sending button removes the activity schedule, stops broadcast queue preparation, and deletes queued messages.
Messages that are already sent or already being delivered cannot be recalled.

When you click Cancel sending, a confirmation prompt appears before any changes are made. Hover text for this button is: "Remove the schedule, stop broadcast queue preparation, and delete queued messages."