Use PowerShell
Install Azure AD modul
Install-Module -Name AzureAD
Get credentials
$UserCredential = Get-Credential
Connect to the service
Connect-MsolService -Credential $UserCredential
Disable the Sync
Set-MsolDirSyncEnabled –EnableDirSync $false
Confirm the new status
(Get-MSOLCompanyInformation).DirectorySynchronizationEnabled
Disable Azure AD Directory Sync without AD Connect
https://morethanpatches.com/2018/07/02/disable-azure-ad-directory-sync-without-ad-connect/