It's worse than that. XL's internal protection is suitable for
preventing inadvertent modification of the workbook.

In addition to the copy/paste workaround, the other steps you've taken
already are trivial to bypass:

  http://www.mcgimpsey.com/excel/removepwords.html

In brief: If someone with the wit to draw breath can access your
workbook, they can access everything in it.

One alternative would be to compile your logic and fixed data into a COM
add-in.

Or perhaps you could send a pdf?

That also won't prevent copying, of course.