Any Excel document can be reverse engineered by a talented Excel
programmer.

The tips here are good ones.

My approach:

1. Make the fonts white so that the worksheet looks empty to a novice
2. Add a sub that makes it so that when someone clicks on a cell that
they get sent to range A1 - That keeps someone from tabbing through the
cells and reading content from the formula bar.
3. Password protect the sheet.
4. Hide the sheet.

Keep in mind. The BEST way to protect data is to never store it in the
Workbook to begin with. Put the sensitive data in a Worksheet that
exists in a plugin.