The simplest solution may be to apply worksheet protection so that your users cannot edit any cells other than the input cells.
You can see how to protect a worksheet in the video lesson: Expert Skills Lesson 7-11 Restrict the cells users are allowed to change.
If preventing editing entirely is too much of an extreme measure, you can use an advisory validation that will warn the user that they shouldn't edit the cell.
You can see an example of this in the video lesson: Expert Skills Lesson 2-8 Create user-friendly messages for validation errors.
I have attached a copy of your workbook with validation rules applied that will display a warning if you attempt to overwrite any of the formulas:
Excel Data Table Sample 9-28.xlsx 12.16KB
I used a formula-driven validation rule with the ISFORMULA function that will show an error message if a user tries to enter a value that is not a formula. This will allow formulas to be edited but not replaced with values.
You can see how to create formula-driven validations in the video tutorial: Expert Skills Use a formula-driven custom validation to enforce complex business rules.
If you would rather that the validation triggered whenever any value is entered you can simply enter =FALSE as the formula.
I hope this is the solution you were looking for, but please feel free to reply if you need any more help with this.
Jonathan is part of the professional team who answer Excel-related questions posted on the ExcelCentral.com forums.
Jonathan also tests our courses prior to publication and has worked on all of our ten world bestselling Excel books for Excel 2007, Excel 2010, Excel 2013, Excel 2016 for Windows and Excel 2016 for Apple Mac. Jonathan has also worked on over 850 video lessons for or video courses covering Excel 2007, Excel 2010 and Excel 2013.
As well as extensive Excel knowledge, Jonathan has worked in the IT world for over thirteen years as a programmer, database designer and analyst for some of the world's largest companies.