Another possible solution: obfuscate (= replace with functionally
equivalent, but very hard to understand) the VBA code within the
workbook.

Invisible Basic is an Excel Add-in for obfuscating the VBA code within
Excel workbooks. Using its "Save Invisibly As" command, you can create
a copy of your workbook whose code is obfuscated. That way, even if
they crack your password, all they will see is really ugly,
inscrutable, code.

It's free and open source and available at:

http://invisiblebasic.sourceforge.net/

Creating a separately compiled component likely is more secure, but
this approach is a lot easier (no separate compiler required). This is
a relatively new Addin and I'd very much appreciate any feedback on the
product the Excel/VBA brain trust associated with this newsgroup could
provide.

John