Do you mean the rectangular knockouts for the breakers? Yes, there are readily available filler plates that snap in these unused openings. If your panel is a Square 'D' brand, the part number is "QOFP" or "HOMFP", depending on whether you have a QO or Homeline panel. Similar filler plates are available from every manufacturer, and are available where you bought your supplies. They're about a buck a piece.
"Nothing is foolproof to a sufficiently talented fool"