Ovviamente non potrai cambiare idea...
codice:
<script>
function togli(obj) {
f = obj.form;
v = obj.options[obj.selectedIndex].value;
for (var i=0;i<f.elements.length;i++) {
  o = f.elements[i];
  if (o==obj) {
    for (var j=0;j<o.options.length;j++) {
      if (o.options[j].value!=v) o.options[j]=null;
    }
  } else {
    if(o.type=="select-one") {
      for (var j=0;j<o.options.length;j++) {
        if (o.options[j].value==v) o.options[j]=null;
      }
    }
  }
}
}
</script>
...
<form>
<select name="s1" onchange="togli(this)">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
... eccetera
</select>
<select name="s2" onchange="togli(this)">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
... eccetera
</select>
<select name="s3" onchange="togli(this)">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
... eccetera
</select>
... eccetera
</form>
ciao