Hi Florian,
Please follow the steps below to disable directory synchronization.
1. Install the Windows Azure AD Module Windows PowerShell.
2. Connect to Windows Azure AD through Windows PowerShell.
3. Type the following cmdlet, and press Enter.
Set-MsolDirSyncEnabled -EnableDirSync $false
4. Type the following cmdlet to make sure that directory synchronization is fully disabled.
(Get-MSOLCompanyInformation).DirectorySynchronizationEnabled
This cmdlet will reture True or False. Continue to run this cmdlet periodically until it returns False.
5. Then the users will be “On Cloud”. This process will take up to 72 hours.
Thanks,
Yang Yu