When you connect your Office 365 calendar to Appointment.one we have access to your calendar only.  This is done using a mechanism called oauth which limits our access.  At no point can we ever access your password or any other part of Office 365 (such as documents, email, etc.).

If you wish to disable Appointment.one's access to your calendar you must change your Office 365 password.  Whenever this is down all authorizations you've made (such as to us) are cleared out.