1. To do this as an Admin, click on the Administration > Advanced > External Calendar Integration section from the main menu of Accudemia.

2. In this list, you can see all tutors that have connected their calendars, similar to this example:

Accounts can be disconnected from Office 365's Calendar by clicking the Disconnect button to the right of the account.
3. You can only disconnect their calendar on this screen.
Conclusion
If you perform these steps, after you can test the appointment scheduling again to see if they still cannot be scheduled during their booking times from their availability. If everything looks good now, then you want to warn them that when connecting their Outlook 365 account they need to be sure that they are not using a calendar that would block off times they should be available for tutoring.