You need to add two entries to the ACL for the object:

• Deny Everyone Delete
• Deny Everyone Delete Tree

I just dug up some really old code to do this that should work:

Dim currentOU as DirectoryEntry ' This is the object you want to protect
Dim deleteAce As New ActiveDirectoryAccessRule(New NTAccount("Everyone"), ActiveDirectoryRights.Delete, AccessControlType.Deny)
Dim deleteSubtreeAce As New ActiveDirectoryAccessRule(New NTAccount("Everyone"), ActiveDirectoryRights.DeleteTree, AccessControlType.Deny)

Dim currentACL As ActiveDirectorySecurity = currentOU.ObjectSecurity

Dim deleteDeny As Boolean = False
Dim deleteSubtreeDeny As Boolean = False

For Each ace As ActiveDirectoryAccessRule In currentACL.GetAccessRules(True, False, GetType(NTAccount))
    If ace.IdentityReference.Value = "Everyone" Then
        If ace.ActiveDirectoryRights = ActiveDirectoryRights.Delete Then
            deleteDeny = True
        ElseIf ace.ActiveDirectoryRights = ActiveDirectoryRights.DeleteTree Then
            deleteSubtreeDeny = True
        End If
    End If
Next

If Not (deleteDeny AndAlso deleteSubtreeDeny) Then
    currentACL.AddAccessRule(deleteAce)
    currentACL.AddAccessRule(deleteSubtreeAce)

    currentOU.CommitChanges()

    Console.WriteLine("Protected: " & currentOU.Path)
End If

You just have to tell Get-ADObject to show that property with the -Properties parameter:

Get-ADobject -Server "myservername+fqdn" `
             -Filter {objectclass -eq "mydominzone"} `
             -SearchBase "DC=DomainDNSZones,DC=aa,DC=bb,DC=cc" `
             -Properties ProtectedFromAccidentalDeletion

Interestingly, there isn't actually any flag in Active Directory for that. All it does is add a "deny delete" permission for "Everyone" to the account. But both Set-ADObject and Get-ADObject translate that into a property that you can set and read. And even AD Users and Computers shows it as a checkbox.