How to lock a cell without protecting the worksheet

I have column B with this formula =IF(C4,WEEKDAY(C4),"") where the reference value goes from C4 to C999
This formula works great to pick the day-of-week

I want to prohibit the user from entering TUESDAY and thus wiping-out my formula

I have several macro's on the sheet / workbook
I have not had any luck in unprotecting-protecting the sheet/workbook within a macro
I guess I've spent 4 hours trying code from this, and other, forums.

So I'm wondering if I can use DATA VALIDATION to do the job?
There is an "in list" function ... is there a way to modify it to be "not in list" ??
-OR-
can I use custom-formula to somehow require what they enter to begin with "=IF(" ??

Thanks
Puzzled-in-St. Louis

I have developed a few forms with this functionality, so it's totally possible.

In one form, I have a up/down arrow object that allows users to change the year for the annual employee review form, which unlocks the sheet, changes the date, relocks the sheet.

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and

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Your variation could be something as simple as:

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Another idea:

You could select C4:C999, choose Data Validation, Text Length, minimum 22, max 26.

Your formula string would have a min of 22 min characters long, and a max of 26 with the row reference for 999.

The only way they could mess it up is typing something silly like MondayMondayMonday (24 characters long)

Excellent and elegantly simple solution without having to mess with macro.

You could setup Worksheet_Change(ByVal Target As Range) to call a sub that adds your formula to the column that you need it to be in. This way if he or she enters als;kjf;lak in the cell, and hits enter... it changes things back to your formula.