You can block the right mouse button in the following way:

  <script language="javascript">
  <!--
     function click(event) {
       if (document.all) {
         if (event.button == 2) {
           return false;
         }
       }
       if (document.layers) {
          if (event.which == 3) {
             return false;
          }
       }
     }

     if (document.layers) {
       document.captureEvents(Event.MOUSEDOWN);
     }

    document.onmousedown=click;
  //-->
  </script>