You could give the cell a custom format so that the cell displays an x no matter
what's typed in the cell--the cell will still contain whatever they type (as
seen in the formula bar), but it'll display an x.

Select the range
format|Cells|number tab|custom category
X;X;X;X

Another option would be to use a worksheet event that replaces anything with an
X.

If you want to try, right click on the worksheet tab that should have this
behavior and select view code.  Then paste this into the newly opened
codewindow:

Option Explicit
Private Sub Worksheet_Change(ByVal Target As Range)
   With Target
       If .Cells.Count > 1 Then Exit Sub
       If Intersect(.Cells, Me.Range("A:a")) Is Nothing Then Exit Sub
       
       On Error GoTo ErrHandler:
       
       If IsEmpty(.Value) Then Exit Sub
       
       Application.EnableEvents = False
       .Value = "X"
   End With
       
ErrHandler:
       Application.EnableEvents = True
       
End Sub

I used any cell in column A, but you can change the range to what you wanted.