Tag selection while adding new post

Just copy paste the following code in your functions.php file which will add some css code in your WordPress back end which will hide the text box and add button of tags meta box to prevent user adding new tags.

add_action('admin_head', 'custom_css');

function custom_css() {
    echo '<style>
#tagsdiv-post_tag .howto,
#tagsdiv-post_tag .tagadd,
#tagsdiv-post_tag #new-tag-post_tag{
    display: none;
    }   
  </style>';
}