Here you go:
Code:
$ADS_UF_PASSWORD_CANT_CHANGE=&40
 
$oUser = GetObject("WinNT://ACME/rhowarth,user")
$oUser.Put("UserFlags",$oUser.Get("UserFlags")^$ADS_UF_PASSWORD_CANT_CHANGE)


Note, you will need a recent version of KiXtart for the XOR "^" operator.