Prevent users from adding taxonomy terms

For these situations build your own metabox for the tax terms and use radio or select.

My fork of Jared Atchison’s Custom Meta Box class supports custom taxonomy metaboxes.

After you get the metabox directory uploaded use this to create your meta box.

$prefix = 'xxx_';  //Add your own unique prefix.
$meta_boxes = array();
$meta_boxes[] = array(
    'id' => 'test_metabox',
    'title' => 'Test Metabox',
    'pages' => array('page'), // post type
    'context' => 'normal',
    'priority' => 'high',
    'show_names' => true, // Show field names on the left
    'fields' => array(
          array(
           'name' => 'Test Taxonomy Radio',
           'desc' => 'Description Goes Here',
           'id' => $prefix.'text_taxonomy_radio',
           'taxonomy' => '', //Enter Taxonomy Slug
           'type' => 'taxonomy-radio',  
        ),
    );

require_once('metabox/init.php');