How can I lock a specific field with a macro?

I have created a document with many fields.

I have prepared an update macro than when run you enter all the information you want to be in each field. All fine up to here.

One of the fields is the FILENAME field which I have placed in the footer so that the filepath of the document is autoupdated. However because the documetn at some point will be stored in a permanent folder I want to add a macro that, when run, will lock that specific field so its not updated anymore.

Any ideas?

regards,

K.