Hi Alex

The default property for all cells on a sheet is Locked.
You would need to
select the cells on a sheet where you want users to be able to enter
data
Format>Cells>Protection>uncheck Locked
Tools>Protection>Protect sheet.

Now the only cells where users can enter anything, is in the cells you
have specifically unlocked.

You might want to do it the other way around, by selecting all cells in
the sheet by pressing the small gray cell above row 1 and left of column
A. This will select the whole sheet. Format all cells as unlocked.
Select your cells with formulae, set property to locked, then protect
sheet.