One aspect of Excel’s protection is the ability to hide the contents of a cell. The cell still displays, but the formula bar remains empty when the cell is selected. This is useful if you don’t want the users to see the formula for the cell.
On the Protection tab of Format>Cells, check the Hidden box.
Then protect the sheet using Tools>Protection>Protect Sheet. With the Hidden property checked and the sheet protected, the formula is no longer visible to the user.
In my line of work I can’t use protected workbooks so instead of hiding the cell I use a custom format(;;;) this hides the text entered on the spreadsheet, however it is still visible in the formula window. It’s a sad compromise I have to make but it has worked for me.