You can purge all undelivered broadcast, funnel, and transactional mail in the backend customer account table.
Click customers -> Customer Accounts
Check the customer account whose mail you want to purge:
Use the action menu to purge mail queues:
The most reliable way to cancel a broadcast in progress is always to purge broadcast mail from the backend customer account menu.
Clicking "Cancel" in the Actions button from the broadcast page will cancel a broadcast sent through Velocity MTA, but may not be able to cancel a broadcast sent through an API or SMTP Relay connection depending on the state of the data transfer at that time.
