Did you create the form in LiveCycle Designer (XFA Forms)? Or in Acrobat (AcroForms)?
The solution I posted is for forms created in LiveCycle Designer.
You will be able to lock fields in AcroForms, but I think you would need to target each field individually (eg a different solution to the one posted). If you have created your form in Acrobat, you may get better advice on the Acrobat forums.
Hope that helps,