codice:
<style>.disable {background-color: silver;color: white;}</style>
<script>var old=-1;</script>
<select 
  onclick="old=this.selectedIndex" 
  onchange="if(this.options[this.selectedIndex].text.indexOf('reserved')>=0) this.selectedIndex=old">
<option value="0">0</option>
<option value="1" class="disable">1 - reserved</option>
<option value="2">2</option>
</select>
Compatibilita' da testare... qui ho solo IE5.5

ciao