It's fairly common knowledge that Excel files do not have "bullet proof" security. The general intent of Excel passwords is to prevent accidental meddling and provide somewhat of an obstacle to intentional meddling. Think of it like locking the door to your home. Is it impregnable when a dishonest person could just break a window and climb in? The intent is to signal to honest people that you don't want them entering certain areas. Any person with fairly unimpressive technical skills could find a way to circumvent your security with only moderate effort.

For real security, development should be in compiled code, as generated by C++, VB, etc.