Recalling emails sent via third party platforms such as Envoke is not technically possible.
Servers only have authority to delete the original message if both users are on the same mail system, for example part of the same Outlook corporate account.
When you send an e-blast, you can unschedule sending any time before the scheduled date/time. Open the scheduled message and click the "Unschedule" link in the message editor.
It's not possible to stop an e-blast when sending is already in progress.