VBA Project Password Stripped when saving as .xlsx

I have an excel file that is distributed within my organization. That file contains certain worksheets that the end-users do not need to see or make changes to, so in the VBA project, I change those files to "VeryHidden". Then I protect the workbook's VBA code by going to Tools->VBAProject- Project Properties-> Protection tab and adding a password. This worked great as it allowed all the other sheets to function as intended, but didn't allow the users to access the sheets that were hidden.

However, we recently upgraded to Excel 2007, and the password protection is stripped whenever the file is saved with the extension .xlsx. I can save it as a "macro-enabled" file with the extension .xlsm, but all that needs to be done to remove the password is to save it with the .xlsx extension again. Is there something that can be done in Excel 2007 to protect VBA code, or am I doing something completely wrong?

This is my first post here, so please let me know if I've done something wrong!