select element of HTML

Try

<script type="text/javascript">
function makeDisable(){
    var x=document.getElementById("mySelect")
    x.disabled=true
}

for more reference and getting practical influence have a go there-[Disable and enable a dropdown list][^]

you can write Javascript. call following function on onchange of select element

function disabledME()
{
  if(document.getElementById("Select1").value == "this")
    document.getElementById("Select1").disabled = true;
}

above code will disabled your select element on selected index changed.