Maybe this can help you.
I created a new workbook, selected all the cells, and UNCHECKED the locked option for all of them.
Next, I protected the sheet, using as password
then, I put this code in the Sheet's module.
Private Sub Worksheet_Change(ByVal Target As Range)
Target.Locked = True
Then, everytime I change a cell, it becomes locked, that means that you can't write on it again.
You could modify them later using Tools | Protect | Unprotect sheet, and supplying the password (Password in this case)