How to Completely Disable Viewstate in ASP.NET

I tried to disable Viewstate of a website by set EnableViewState Page directive to false.
But when I get the Page HTML Source I found the following line:

<input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE"
 value="/wEPDwUKMTY1NDU2MTA1MmRk7eWz8DBE7zF7MW8sS4wNndKCr5gBJPZZ/LOS1KIbBo4=" /> 

protected override void SavePageStateToPersistenceMedium(object viewState)
{

}

protected override object LoadPageStateFromPersistenceMedium()
{
  return null;
}