IF + Manual

Lucassis 

You cannot do that with a formula; it would require VBA. If that is OK: let's say you want to apply this to cells B1:B10 - if the corresponding cell in A1:A10 equals 1, enter "X", otherwise let the user enter data.

Right-click the sheet tab.

Select 'View Code' from the context menu.

Enter the following code into the worksheet module:

Private Sub Worksheet_Change(ByVal Target As Range)
    Dim rng As Range
    If Not Intersect(Range("A1:A10"), Target) Is Nothing Then
        Application.EnableEvents = False
        For Each rng In Intersect(Range("A1:A10"), Target)
            If rng.Value = 1 Then
                rng.Offset(0, 1).Value = "X"
            End If
        Next rng
        Application.EnableEvents = True
    End If
End Sub

Switch back to Excel.

Select B1:B10.

On the Data tab of the ribbon, click Data Validation.

Select Custom from the Allow drop-down.

Enter the formula

=OR(A1<>1,B1="X")

Activate the Error Alert tab.

Enter an appropriate error message to be displayed if the user tries to enter something else than "X" if column A contains 1.

Click OK.

Save the workbook as a macro-enabled workbook (.xlsm).

Makes sure that you allow macros when you open it.