How to hide a in a menu with CSS?

I would suggest that you do not use the solutions that use a <span> wrapper because it isn’t valid HTML, which could cause problems down the road. I think the preferred solution is to actually remove any options that you wish to hide, and restore them as needed. Using jQuery, you’ll only need these 3 functions:

The first function will save the original contents of the select. Just to be safe, you may want to call this function when you load the page.

function setOriginalSelect ($select) {
    if ($select.data("originalHTML") == undefined) {
        $select.data("originalHTML", $select.html());
    } // If it's already there, don't re-set it
}

This next function calls the above function to ensure that the original contents have been saved, and then simply removes the options from the DOM.

function removeOptions ($select, $options) {
    setOriginalSelect($select);
    $options.remove();
 }

The last function can be used whenever you want to “reset” back to all the original options.

function restoreOptions ($select) {
    var ogHTML = $select.data("originalHTML");
    if (ogHTML != undefined) {
        $select.html(ogHTML);
    }
}

Note that all these functions expect that you’re passing in jQuery elements. For example:

// in your search function...
var $s = $('select.someClass');
var $optionsThatDontMatchYourSearch= $s.find('options.someOtherClass');
restoreOptions($s); // Make sure you're working with a full deck
removeOptions($s, $optionsThatDontMatchYourSearch); // remove options not needed

Here is a working example: http://jsfiddle.net/9CYjy/23/