Within either the 'Contacts' or 'Speakers' module, select the contact(s) you would like to remove portal access from.
At the top of your page select the button 'More' > 'Manage Portal Access'.
From the pop-up window, select 'Remove Access' for the contacts you would like to revoke access from.
NOTE: This is a manual process as you will have to select 'Remove Access' from each individual contact.
Updated over a week ago