Use Union when you want to generate a range from two or more blocks of cells.
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
Dim rngForbidden As Range
Set rngForbidden = Union(Range("B10:F20"), Range("H10:L20"))
Range("A1").Select
MsgBox "You can't select cells in " & rngForbidden.Address, vbCritical
End Sub
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