@VPLunn22 Thanks for reaching out.
1) If you do not want to sync anything from on-premises and want to delete whatever has synced from on prem to azure AD
You need to delete those user accounts from on-premises let that change sync to Azure AD via azure AD connect, this will remove all the objects which were synced from onprem AD.
2) If you have already uninstalled azure AD connect and want to remove the accounts which got already synced.
You need to disable the directory synchronization first, this will make all the user which were synced to AzureAD as Cloud only accounts and you will be able to delete them from portal or from powershell.
3) Accounts created directly on Azure AD (Cloud only accounts)
These type of accounts will not have any effect for your process, they will continue to authenticate and function normally with no data loss.
Let me know if you have any further question.
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