type
  TEvilClass = class
  public
    constructor Create;
  end;

  TGoodClass = class(TEvilClass)

  end;

{ TEvilClass }

constructor TEvilClass.Create;
begin
  if ClassType = TEvilClass then
    raise Exception.Create('I''m the evil class which cannot be instantiated!');
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  EvilClass: TEvilClass;
begin
  EvilClass := TEvilClass.Create;
end;

procedure TForm1.Button2Click(Sender: TObject);
var
  GoodClass: TGoodClass;
begin
  GoodClass := TGoodClass.Create;
end;