You could password protect the VBA code - but there is software around that will crack this so Excel is never 100% secure
I recently wrote an algorythm - where the password required by the user is actually calculated in VBA - so the user has to ask for a password from the administrator that would then be valid for an hour or so. This turned out to be quite useful as the administrator was then in total control of all access to the workbook. This can still be cracked though, but the code would be tough to find and follow through. Software to crack VBA is less available than software to crack workbooks/worksheets

If you want to know how the author of your workbook built the protection, e-mail me off-list and I'll give you an address you can send the workbook to where I'll open it up and see whats going on.