How do I prevent deleted contacts from registering again?

Set the status of the contact to "unknown" and then delete it.

Why you have to change the status first: When a contact is deleted on Quentn, the system remembers the status of the email address (based on an anonymous hash value). If the contact registers again, it will immediately get its old status back. If this status is "unknown", the contact will also receive opt-in emails again.

This measure serves to protect us from fraud. For example, if someone has unsubscribed or marked our emails as spam, the system administrator should not be able to change this status again - not even by deleting and re-creating the contact.