Track portal usage in order to manage portal user list

As users at our customers move on, we are unlikely to be updated to say that they have moved on. However, for data protection and security purposes we need to be able to remove customer users from the portals when they are no longer using it. It would be helpful to be able to view a report that showed when a portal user had last logged in as voting/adding ideas and commenting alone is not enough to indicate portal activity.