procedure TForm1.RichEdit1KeyPress(Sender: TObject; var Key: Char);
begin
  if Key = chr(VK_TAB) then
  begin
    beep;
    Key := #0;
  end;
end;

procedure TForm1.RichEdit1KeyDown(Sender: TObject; var Key: Word;
  Shift: TShiftState);
var
  msg: TMsg;
begin
  if Key = VK_TAB then
  begin
    beep;
    while PeekMessage(msg, RichEdit1.Handle, WM_KEYFIRST, WM_KEYLAST,
      PM_REMOVE) do;
  end;
end;