A cloud-based identity and access management service for securing user authentication and resource access
I understand that you would like to stop synchronizing and creating the users from on-premises AD to Microsoft Entra ID and manage all the users from Entra ID.
On premises users are synced to Entra ID by using the Entra connect Sync or Entra cloud sync tool and the users are get created by admin or any HR application provisioning.
If you have HR application provisioning to create the users, you need to configure the HR application to provision in Entra ID.
If you want to remove the users from Active directory and mange under Microsoft Entra ID only you need to convert the user from on-prem users to cloud only users, as of now there is no option available directly to convert the on-premises user to cloud only users.
however, you can achieve this by create an OU in on-premises Active directory and make this OU is out of sync scop in your Entra connect tool, once you create the OU move the targeted users from sync OU to non- sync OU, on the next sync cycle the user will be deleted in Microsoft Entra ID.
The deleted users are in soft delete state in Entra, so that you can restore it from deleted users, once you restored the users became cloud only users.
If you have bulk users and your organization wants to move entirely to the cloud, you need to disable the synchronization in both on-promises and Entra ID. before doing this stop creating the new users and groups and cleanup the unassay groups and users and admin accounts from AD.
Please follow How Disable On-Premises Directory Sync to Microsoft Entra ID
Hope this helps. if my suggestion is not helpful, please let me know by responding in the comments section.
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