Modifies the behavior of the taxonomy box, forbids user from selecting terms that don't belong to taxonomy.

no-new-terms-taxonomy-meta-box.php

<?php
/*
Plugin Name: No new terms taxonomy meta box
Plugin URI: https://gist.github.com/1074801
Description: Modifies the behavior of the taxonomy box, forbids user from selecting terms that don't belong to taxonomy.
Author: Hameedullah Khan
Author URI: http://hameedullah.com
Version: 0.1
License: Do what ever you like, but don't publish it under your name without improving it.
 */

/*
 * For more information: http://wordpress.stackexchange.com/questions/20921/
 */

// currently works only with single taxonomy which should be defined here
// default is the built-in post_tag
define('CTM_TAXONOMY_NAME', 'post_tag');

function ctm_custom_tax_js() {

	// taxonomy name not defined or set to empty value
	if ( !defined('CTM_TAXONOMY_NAME') || !CTM_TAXONOMY_NAME ) {
		return;
	}
?>
<script type="text/javascript">


	function ctm_custom_termadd_handler(event){
			var tax = '<?php echo CTM_TAXONOMY_NAME; ?>';
			var input = jQuery('#tagsdiv-<?php echo CTM_TAXONOMY_NAME; ?> input.newtag');

			var q = input.val().split(',');

			// if there are more then two values, just add the first one
			// NOTE: because this solution does not support inserting multiple terms
			if (q.length > 1) {
				q = jQuery.trim(q[0]);

				// as we don't support multiple terms
				// set the value of input box to the first term
				input.val(q);
			}

			jQuery.get( ajaxurl + '?action=ajax-tag-search&tax=' + tax + '&q=' + q, function(results) {
				var tokens = results.split('\n');
				for (var i=0; i < tokens.length; i++) {
					token = jQuery.trim(tokens[i]);
					if ( token && token == q ) {
						(function($){
							tagBox.flushTags( $('#tagsdiv-<?php echo CTM_TAXONOMY_NAME; ?>') );
						})(jQuery);

						return true;
					}
				}

			} );
			event.stopImmediatePropagation();
			return false;
	}

	function ctm_custom_key_handler(event) {
		if (13 == event.which) {
			ctm_custom_termadd_handler(event);
			return false;
		}
		return true;
	}

	jQuery(document).ready(function() {
		// unbiind the click event from the taxonomy box
		jQuery('#tagsdiv-<?php echo CTM_TAXONOMY_NAME; ?> input.tagadd').unbind('click');
		jQuery('#tagsdiv-<?php echo CTM_TAXONOMY_NAME; ?> input.newtag').unbind('keyup');

		// hide the howto text for inserting multiple terms
		// NOTE: because this solution does not support inserting multiple terms
		jQuery('#tagsdiv-<?php echo CTM_TAXONOMY_NAME; ?> p.howto').hide();

		// bind our custom handler
		jQuery('#tagsdiv-<?php echo CTM_TAXONOMY_NAME; ?> input.tagadd').click(ctm_custom_termadd_handler);
		jQuery('#tagsdiv-<?php echo CTM_TAXONOMY_NAME; ?> input.newtag').keyup(ctm_custom_key_handler);
	});

</script>

<?php
}
add_action('admin_footer-post-new.php', 'ctm_custom_tax_js');
add_action('admin_footer-post.php', 'ctm_custom_tax_js');
?>