<form action="">
<select disabled>
<option value="Test">Test</option>
</select>
<input type="radio" name="ja" onclick="if(this.checked == true){document.forms[0].elements[0].disabled=false} else {document.forms[0].elements[0].disabled=true}">
anschalten
<input type="radio" name="ja" onclick="if(this.checked == true){document.forms[0].elements[0].disabled=true} else {document.forms[0].elements[0].disabled=false}">
ausschalten
</form>