How to disable manual input for JQuery UI Datepicker field?

I decided to use the JQuery UI Datepicker script for picking dates. Below is part of my code, and the way I integrated it into my PHP page:

<link type="text/css" href="css/south-street/jquery-ui-1.8.6.custom.css" rel="stylesheet" />
<script type="text/javascript" src="js/jquery-1.4.2.min.js"></script>
<script type="text/javascript" src="js/jquery-ui-1.8.6.custom.min.js"></script>
<script type="text/javascript">
$(function(){
// Datepicker
$('#datepicker').datepicker({ dateFormat: 'yy-mm-dd' });
//hover states on the static widgets
$('#dialog_link, ul#icons li').hover(
function() { $(this).addClass('ui-state-hover'); },
function() { $(this).removeClass('ui-state-hover'); }
);
});
</script>

And:

    // date picker (embeds JQuery script)
echo '<label for="datepicker">Date: </label>';
echo '<input type="text" name="datepicker" id="datepicker" />';
echo '<div id="datepicker"></div>';

Pretty much according to the JQuery UI instructions.

Now my question is: how can I disable manual inputs in the text input field? I only want the JQuery Datepicker script to be able to fill the text field. As far as I can see, the script currently prevents the user from entering any non-numerical characters into the text field, but any combination of numbers is allowed (therefore, gibberish like "95460829468" is allowed).