Simply create a very hidden
name, Expired, referring to
Code:
=(1/(NOW()<DATE(2002,2,1)))
(for example), and
add Expired to all your formulas. Then hide your formulas while leaving
their results visible (check Hidden in the Protection tab in the Format
Cells dialog) before protecting the worksheet. Even this isn't that much of
a barrier since it's very easy to defeat password protection in Excel.