If you would like to remove a portal user, you can do it from the Client Group view.

First, locate the user via the search box by entering their email address or scroll down to the portal they have access to if you remember it. Then click on the '...' icon asssociated with their portal, go to 'manager users' and remove them from the list.

If a user has access to many portals, you need to repeat that action for each portal they have access to.