const
  WM_AFTER_SELECTOBJECT = WM_USER + 400;

private
procedure WmAfterSelectObject(var Msg: TMessage); message WM_AFTER_SELECTOBJECT;

procedure TForm1.ImageEnVect1NewObject(Sender: TObject; hobj: Integer);
begin
  { Post the custom message WM_AFTER_SELECTOBJECT to our form }
  PostMessage(Self.Handle, WM_AFTER_SELECTOBJECT, 0, 0);
end;

procedure TForm1.WmAfterSelectObject(var Msg: TMessage);
var
  i: Integer;
  iHObj: Integer;
begin
  iHObj := -1;
  with ImageEnVect1 do
  begin
    for i := -1 to SelObjectsCount - 1 do
    begin
      if i >= 0 then
        iHObj := SelObjects[i];
      if iHObj <> -1 then
        if ObjKind[iHObj] = iekPOLYLINE then
          UnSelObject(iHObj);
    end;
  end;
end;