Now, I know that it is impossible to lock a document that contains mail
merge.  I also read that if I change the document into a template I can do it
that way.  Here's my problem:

We have a shared letter than uses mail merge.  My coworkers keep deleting
fields in the letter itself and then wonder why it doesn't have all the
information.  Is there a way that they can add/change the data source without
them being able to change the mail merge document itself?  (I hope I got all
of the terminology correct)  

These folks won't consider learning how to open the template through a new
document, they will just open and change the template.  Most of them aren't
too savvy when it comes to computers (really, neither am I, so use small
words please  :), and I'm getting sick of fixing the mail merge document.  
Hence, why I need them to have access to the document without changing
anything besides the records.

We use Microsoft Office Word 2003 for all of our word processing needs.