type
  TForm2 = class(TForm)
    { другой код выше }
    procedure CreateParams(var Params: TCreateParams); override;
    { другой код ниже }
  end;

procedure TForm2.CreateParams(var Params: TCreateParams);
begin
  inherited CreateParams(Params);
  Params.Style := Params.Style and not WS_OVERLAPPEDWINDOW or WS_BORDER
end;