Drop down selection - Do nothing on certain selctions

I've a drop down menu, but when you selct one of the headings, it takes you off into no where, how can I prevent this.

My code is:

<script language="JavaScript">
<!--
function MM_jumpMenu(targ,selObj,restore){ //v3.0
  eval(targ+".location='"+selObj.options[selObj.selectedIndex].value+"'");
  if (restore) selObj.selectedIndex=0;
}
//-->
</script>

Code (markup):

and

<form name="alquiler">
<select name="menu1" onChange="MM_jumpMenu('parent',this,0)">
<option selected>* Menu *</option>
<option value="http://www.site2com"></option>
<option class="dropdown">* [b]DO NOTHING HERE[/b] *</option>
<option value="http://www.site3com"></option>
</select>
<input type="button" name="Button1" value="Go" onClick="MM_jumpMenuGo('menu1','parent',0)">
</form>

Code (markup):

 

Try value="#"