how can I protect entries in a shared workbook?
 

I have a shared workbook so that many users can write simultaneously. I want
the cells to be protected in a way that once a cell has been modified for the
first time (from blank) it cannot be modified again by another user.
In other words, if a cell is blank, it can be modified by anyone, if it is
not blank it should prompt for a password in order to be modified.