Don't Associate Orders With Drupal Users - Guest Checkout Only
It sounds like you're trying to work against some basic assumptions of Drupal Commerce, namely that users who are logged in should be associated to their data. This is true of Drupal itself - for example, an authenticated user cannot create an anonymous comment.

The simplest thing to do would be to instruct these users to log out. You can likely find a way to alter how Drupal Commerce creates new shopping cart orders such that authenticated users get anonymous orders associated to their sessions similarly to anonymous users, but I can't guarantee that the module throughout won't be checking to see if the user is currently logged in and making assumptions based on this elsewhere.