seems your proposal to protect the excel  files will be
useless....since it is obvious that macro  (vba excel) is kinda open
source file....

if you can protect to open the file, but after the user  open the file,
they can see and copy your listing program....

i don't want to discourage, but in my experience we can't protect the
listing program in excel perfectly.... if other have any opinion feel
free to correct me