To remove users from a portal, navigate to the People tool and select the user you would like to remove. Locate the Remove from Portal option on the right side of the page and confirm the action. Refresh the page to confirm the change worked as expected.
To mass remove users, click on one user, hold down shift, then select another user on the list. This will select both users and everyone in between. To select multiple users and not everyone in between, click on a user, hold down control (or command) and click on another user. This will only include the users you've selected. Next, choose Remove on the right side of the page and select Remove from Portal.