A list of all the users can be found in the side bar under Settings->Users, here you can select which user you would like to review for deletion. You can use the search filter in the top right to find your results faster.
User_list.png
After you have selected a user, at the bottom of the page you will see a 'Delete' button, click on this and you will be prompted to confirm your decision. Click 'OK' and the user will be removed from the portal.
