To stop the sending of a phish that has been published, navigate to the Phish in Progress section:
Open the Phish by clicking on View Phish:
Click on the blue Archive button:
This will 'archive' the phish, i.e. no more emails will be sent, and no more data will be collected from users who had already received the phish.
Reports will still be available for a phish in an archived state.