Unfortunately, Excel is not a secure environment. Worksheet/Workbook/VB protection can be cracked within seconds. Of course, less than 1% of Excel users would know how to do so (or even want to).
You could look into converting your Excel file into an EXE file, or possibly creating your "application" in another language (.Net, for example) and build in Excel-like functionality for data entry/review. With the right knowledge of programming and databases, you can create quite complex - and secure - Office applications.