How can I prevent users from joining my existing Office 365 tenant?
There are steps you can take, as an admin, to prevent users from joining your existing Office 365 tenant. If you do block this, users’ attempts to sign up will fail and they will be directed to contact their organization’s admin. You do not need to repeat this process if you have already disabled automatic license distribution (e.g. Office 365 for Education for Students, Faculty, and Staff).
These steps require the use of Windows PowerShell. To get started with Windows PowerShell, see the PowerShell getting started guide.
To perform the following steps, you must install the latest 64-bit version of the Azure Active Directory Module for Windows PowerShell.
After you select the link, select Run to run the installer packag