How do I delete a contact permanently?
You can delete a single contact from its detail page permanently. To delete a particular contact from the contact details page:
Click Contacts from the Navigation Toolbar to the left. Select View Contacts.
Click on a particular contact to view its details.
Click the More Actions button at top-right corner of the contact's profile picture.

Choose Delete.
In the pop-up that appears, you can enable the Delete Permanently checkbox and click the Delete Permanently button.

Once a contact is permanently deleted, you can't manually re-add it from your end. If you still wish to re-add that particular contact, they have to add their details via a signup form, a pop-up, or other means.