To stop a predictive email from sending after a contact has already passed through a "Predictive Email send" action in an automation, you can do one of the following:
Delete the Predictive send block in the automation
Set the automation to inactive - but they will need to leave it inactive for 24 hours, so all queued contacts drain from the system
Delete the automation
To stop the email from sending to a specific contact, you need to delete that contact.