- Right click on the cell that you want to lock
- Navigate to the protection tab
- Make sure the "Locked" box is checked if you want the formula to be uneditable.
- Make sure the "Hidden" box is checked if you also want to not display the formula
Then select OK, go to the Review Tab in excel, select protect sheet, enter your password and that's about it.
Let me know if you have any problems. Not sure which version of Excel you are using, they vary slightly.