What follows is a macro that locks merge fields. If you get rid of the test
               If oField.Type = wdFieldMergeField Then
                   oField.Lock
               End If

and substitute
                   oField.Lock

it should work. Haven't tested though.

oStory.Fields.Lock
might lock all fields in the story. I expect it would, but haven't tested.

Sub MergeFieldLockAllStory()
'   Written by Charles Kyle Kenyon 1 April 2005
'
'   All Story Field Locker - Merge fields
   Dim oField As Field
   Dim oStory As Range
'    On Error Resume Next
   For Each oStory In ActiveDocument.StoryRanges
   ' This goes into headers and footers as well as the regular document
       Do
           For Each oField In oStory.Fields
               If oField.Type = wdFieldMergeField Then
                   oField.Lock
               End If
           Next oField
           Set oStory = oStory.NextStoryRange
       Loop Until oStory Is Nothing
   Next oStory
End Sub

Hope this helps,

I don't know of an automatic way to do this, but the following code will do
it --

Sub Macro1()

Dim pRange        as Word.Range

For each pRange in ActiveDocument.Storyranges
   Do
       pRange.Fields.Unlink
       set pRange = pRange.NextStoryrange
   Loop until pRange is nothing
Next

Msgbox "Finished"

End Sub

If the document is collated via IncludeText fields, there is a switch to say
not to update fields within them.  Not sure if the same switch works for
other types of fields--check in Help.

See #4 "Fields within Fields" here:
http://word.mvps.org/FAQs/TblsFldsFms/includetextfields.htm