Is it possible to disallow free text entry in the JQuery UI autocomplete widget?

eg I only want the user to be allowed to select from the list of items that are presented in the autocomplete list, and dont want them to be able to write some random text.

I didn't see anything in the demos/docs describing how to do this.

http://jqueryui.com/demos/autocomplete/

I'm using autocomplete like this

$('#selector').autocomplete({
    source: url,
    minlength: 2,
    select: function (event, ui) {
        // etc
    }