type
TMonThread = class(TThread)
  private
    procedure Input;
  protected
    procedure Execute; override;
  public
    Stopped : Boolean;
    constructor Create;
    destructor Destroy; override;
    procedure DoThread(Status : Boolean);
    procedure Stop;
  end;
{...}
procedure TMonThread.DoThread(status : Boolean);
var
  Overlapped: TOverlapped;
  Signaled : DWORD;
begin
Signaled:=0;
if ComPort2.InstanceActive then
  SetCommMask(ComPort2.Handle, EV_RXCHAR) else
    begin
    Form1.Memo_sms.Lines.Add('SetCommMask not set');
    exit;
    end;
//set waiting mask
  FillChar(Overlapped, SizeOf(Overlapped), 0);
  Overlapped.hEvent := CreateEvent(nil, True, True, nil);
  WaitCommEvent(ComPort2.Handle, Signaled , @Overlapped);
  repeat //<--------------------------------------
      sleep(readint);
      Form1.Memo_sms.Lines.Add(IntToStr(Signaled));
      Application.ProcessMessages;
  until (Signaled = EV_RXCHAR) or (Stopped = True); //<----------
  if (Signaled = EV_RXCHAR) then
    begin
    Form1.Memo_sms.Lines.Add('Input!');
    Input;
    end;
end;
{...}
procedure TMonThread.Stop;
begin
Form1.Memo_sms.Lines.Add('Thread Destroyed');
// clear buffers
SetCommMask(ComPort2.Handle, 0);
PurgeComm(ComPort2.Handle, PURGE_TXCLEAR or PURGE_RXCLEAR);
//stop thread
Stopped:= True;
end;

destructor TMonThread.Destroy;
begin
inherited Destroy;
end;

procedure TForm1.s_B_disconnectClick(Sender: TObject);
var
  InputThread : TMonThread;
begin
//stop Thread & destroy
InputThread.Stop; //<---------------------------------
//purge buffers of port
if ComPort2.InstanceActive then
begin
ComPort2.Flush;
ComPort2.CloseSocket;
end;