Seriously: Encrypt the file with something like PGP and don't let anyone
near your hard drive.
If someone can open your workbook, they can copy it. If you search the
archives of these newsgroups you'll find lots of suggestions for making
it harder, but nearly all of them can be defeated by any moderately
curious user who can find these newsgroups.
Using Windows/Office Digital Rights Management can also make it harder,
but it requires that your users are willing to use it as well.
I will add to JE's comment - if a user can see your file then can use a
screen capture program to copy the data into another program, in effect
bypassing any protection you have devised.
So, very simply, if they can see it they can copy it.
