If a session has multiple users assigned to it, one can use the edit slot (hand with pen) link and then amend the list of assigned users at the bottom of the slot settings page. However, while it seems like you can delete all of them one by one using the X, upon saving and returning to scheduler summary page, the last user will still be remaining, and it seems impossible to delete them this way. The only way to clear everyone out is to use the 'revoke' link (thumb down icon).