You can use powershell to turn off directory synchronization between AD-Onpremises and Office 365.
To turn off directory synchronization :
Open Windows Azure Active Directory Module for Windows Powershell ( first, you need to install, you can download it, refer to this following link https://technet.microsoft.com/en-us/library/dn975125.aspx ).
Connect to Office 365 Powershell, Type Credential user & password Office 365 :
$UserCredential = Get-Credential
Import session Msolservice and type credential user & password office 365 :
And run this command to turn off dirsync :
Set-MsolDirSyncEnabled -EnableDirSync $false