If someone has left your organization, it's good practice to block their account. Don't ever delete them, though. If they created any content, it will go into limbo! 

Since most websites are configured to disallow anonymous users from creating content, once you delete someone their content reverts to being created by Anonymous ... and therefore disappears. It's not un-fixable, but you'll have to ask us for help and it will take some time.

So, instead:
  • Go to Commands > Manage Users
  • Find the user you would like to block, click the "edit" button to the left of their name
  • Scroll down and change their status from "Active" to "Blocked"

  • Scroll down and save!  

This will end their ability to access the internal part of the site. They will not be able to log in, and therefore cannot edit / create / delete content.