Edit in JSFiddle

$(document).ready(function(){
    $('#btnDisableByVal').click(function(){
       $("#ddlList option[value='2']").attr("disabled","disabled");
    });
    
    $('#btnDisableByText').click(function(){
       $('#ddlList option:contains("HTML")').attr("disabled","disabled");
    });
    
     $('#btnEnableByVal').click(function(){
       $("#ddlList option[value='2']").removeAttr("disabled");
    });
    
    $('#btnEnableByText').click(function(){
       $('#ddlList option:contains("HTML")').removeAttr("disabled");
    });
});

<select id="ddlList">
  <option value="1">ASP.NET</option>
  <option value="2">C#</option>
  <option value="3">VB.NET</option>
  <option value="4">HTML</option>
  <option value="5">jQuery</option>
</select>

<br/><br/>
<input type="button" id="btnDisableByVal" Value="Disable Item By Value" />
&nbsp;&nbsp;
<input type="button" id="btnDisableByText" Value="Disable Item By Text" />
<br/><br/>
<input type="button" id="btnEnableByVal" Value="Enable Item By Value" />&nbsp;&nbsp;
<input type="button" id="btnEnableByText" Value="Enable Item By Text" />