There are 3 simple ways that I know of to protect formulas. (Not including VBA!)
1. Right click the range and select Format Cells, Protection then tick the check
box marked Locked.
2. When saving a file, go to File, Save As and select Options or General Options (depending on your Excel version) There are password options there that you can use.
3. Go to Data, Validation, select Custom, then enter ="".
Please note that protection features for Excel are not 100% reliable, use them with caution.