You can change the access and role from both current employees and past employees:
  1. Go to Core > Users > Edit user profile data
  2. Enter the name and click Search
  3. Click on the name
  4. Under System Information, select Employment
  5. Select Employ in the top right corner
  6.  Mark the button Effective Immediately and mark one of the school levels
  7. Click  Save & Exit
To withdraw a user, follow steps 1 - 5, then
  1. Scroll down to the bottom of the page and enter the Effective Date/Time of withdrawal
  2. Mark Disables the user's login credentials 
  3.  Save & Exit