Autoriser écriture que si première cellule vide selectionné.

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
Const col$ = "C"
  If Not Intersect(Target, Me.Columns(col)) Is Nothing Then
    Call Proteger(col)
  End If
End Sub
Private Sub Proteger(col As String)
Dim rng As Range
Dim cel As Range
  With Me
    .Unprotect
    .Cells.Locked = False
    .Columns(col).Locked = True
    Set cel = .Cells(.Rows.Count, col).End(xlUp)
    If cel.Formula <> "" Then Set cel = cel.Offset(1)
    cel.Locked = False
    .Protect
  End With
End Sub