procedure TForm1.Button1Click(Sender: TObject); 
var 
  M: TMenu; 
begin 
  M := Application.MainForm.Menu; 
  // Den ersten Menueintrag deaktivieren 
  // The first Item will be disabled 
  EnableMenuItem(M.Handle, M.Items[0].Command, MF_BYCOMMAND or MF_DISABLED); 
end;