You could use the Cell Change event to change the font to a 'Webdings' font after the user changes the cell. Then all anyone needs to do is select the cell and view the text unaltered in the formula bar... So you could turn off the formula bar, but then anyone could copy the text and view it in a clip-board viewer to see whats typed. Like Dave suggested, you might consider using a Textbox or Textbox on a userform. With the Textbox you can set a Mask option, and select the caracter to use as the mask character.
Is there a way to only save the contents of textbox without saving the whole sheet.
Have the Textbox value pull from another workbook, .INI, or text file. Either read/write values from the external file or use a hidden workbook to store the password. Then you can save the password changes without saving the active workbook. This of course requires different coding to refer to data in a separate file or workbook. A little more complex.