jQuery/Materialize: Changing select option back to disabled select on clear

You have to reset both select element and material select:

var select = $('select');

$(".btn").click(function(){
    $("form input").val("");

    select.prop('selectedIndex', 0); //Sets the first option as selected
    select.material_select();        //Update material select
});

select.material_select();

You have to select that select the input to the none value and then re render the component:

$(".btn").click(function(){
   $("form input").val("");
   $("select").val("None");
   $('select').material_select();
});
$('select').material_select();

Here is a fiddle with it https://jsfiddle.net/xjqgeuhp/