How can I block Java updates, network-wide?

To disable the auto-update notification you should be able to set the dword value at HKLM\SOFTWARE\JavaSoft\Java Update\Policy\EnableJavaUpdate to 0 and HKLM\SOFTWARE\JavaSoft\Java Update\Policy\NotifyInstall to 0.

Here is a fragment from on of my startup scripts.

Option Explicit

Dim oShell
Set oShell = WScript.CreateObject("WScript.Shell")

DisableJavaUpdateNotification()

Function DisableJavaUpdateNotification
    Dim Key,Value
    Key="HKLM\SOFTWARE\JavaSoft\Java Update\Policy\EnableJavaUpdate"
    Value="0"
    oShell.RegWrite Key,Value,"REG_DWORD"
    Key="HKLM\SOFTWARE\JavaSoft\Java Update\Policy\NotifyInstall"
    Value="0"
    oShell.RegWrite Key,Value,"REG_DWORD"
End Function