selection de plage

Option Explicit
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Target <> "" Then
    MsgBox "LA CELLULE EST DEJA RENSEIGNEE, VOUS NE POUVEZ PAS LA MODIFIER. "
    Range("A1").Select
End If
End Sub

Option Explicit
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
Dim Cellule As Range, BoolRemp As Boolean
    If Not Application.Intersect(Target, Range("a3:j65000")) Is Nothing Then
        For Each Cellule In Application.Intersect(Target, Range("a3:j65000"))
            BoolRemp = False
            If Cellule <> "" Then
                BoolRemp = True
                Exit For
            End If
        Next Cellule
        If BoolRemp Then
            MsgBox "LA CELLULE EST DEJA RENSEIGNEE, VOUS NE POUVEZ PAS LA MODIFIER. "
            Range("A1").Select
        End If
    End If
End Sub

Option Explicit
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
Dim c As Range
For Each c In Target
    If c <> "" Then
        MsgBox "LA CELLULE EST DEJA RENSEIGNEE, VOUS NE POUVEZ PAS LA MODIFIER. "
        Range("A1").Select
    End If
Next c
End Sub