As a Platform Manager: 
  1. Navigate to Core.
  2. Select Users/Access > Profile > Manage Roles.
  3. Click on the role you want to make active/inactive.
  4. To the right of Role Properties, click Edit.
  5. Check the Active box to make the role Active, or un-check the Active box to make it Inactive.
  6. Click Save & Close.
User-added image

Note: A role must be Active to show in Role drop downs, such as through Core > Users/Access > Profile > Edit User Profile Data, or Core > People Finder.