[BETEPAH]

<form name="myform">
  <input type="checkbox" name="tor" onclick="n()"/>
</form>
<script>
function n() {
  var a2 = 0;
  if ((document.myform.tor.checked == true) && (a2 == 0)) {
    document.myform.tor.checked = false;
    alert('Эти вещи не стыкуются!');
  }
}
</script>