Formulas

You can easily achieve this by protecting the whole worksheet (Tools > Protection > Protect Sheet).
The disadvantage of this though is that once a worksheet is protected useful features (such as ????) become unavailable.
This example shows you an alternative to protecting the worksheet in order to protect your formulas.

Find the correct formula

Activate the Data Validation dialog box

Highlight the cells that contain formulas you would like to protect.
You can select non-contiguous cells by holding down the Ctrl key and selecting the individual cells.
Press (Data > Validation)
In the "Allow" drop-down box, select "Custom"

Enter the formula

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Input Message and Error Alert tabs

It is always worth entering a helpful message into both of these tabs.
Input Message - This message will be displayed when the cell is selected (before the users enters any data). This message should be a polite reminder that the data is restricted and should give the user an indication as to what data is allowed.
Error Message - This message will be displayed if the user enters data that does not conform to the validation rules that have been set up.