procedure TForm1.Button1Click(Sender: TObject);
begin
  EnableMenuItem(GetSystemMenu(Handle,false), SC_CLOSE, MF_BYCOMMAND or MF_DISABLED);
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
  EnableMenuItem(GetSystemMenu(Handle,false), SC_CLOSE, MF_BYCOMMAND or MF_ENABLED);
end;