This seems to work:
$("#theSelect").change(function(){
var value = $("#theSelect option:selected").val();
var theDiv = $(".is" + value);
theDiv.slideDown().removeClass("hidden");
//Add this...
$("#theSelect option:selected").attr('disabled', 'disabled');
});
$("div a.remove").click(function () {
$(this).parent().slideUp(function() { $(this).addClass("hidden"); });
//...and this.
$("#theSelect option:disabled").removeAttr('disabled');
});