In our work, we came up very specific functional requirement for excel spreadsheet we maintain.
We need one of the column to have following properties:

- All cells in the column must have unique values (will use some mathematical hash function to do it, so this is not core of question here)
- Only empty cells in this column can be edited and it can be edited only once.
- Once it has value, it can not be edited at all. Not under any circumstances.

Is it possible, does it need VBA programming or can be accomplished without programming? Thanks.,