How to customize the Categories meta box to allow only one category?

Question

I'm trying to customize the Categories meta-box when adding a post.

I have added a custom post and I need the administrator to post a new post without the ability to use the top level categories and also, he can just choose only one category, no more than one category!

Answer 1

Freed from comment; the asker really did not want to write an answer. :/

function convert_root_cats_to_radio()
{
    global $post_type; 
    ?> 
<script type="text/javascript"> 
jQuery("#activitycategorychecklist>li>label input").each(function(){ 
    this.disabled = "disabled"; 
}); 
jQuery("#activitycategorychecklist>li>ul>li>label input").each(function(){ 
    this.type = 'radio'; 
}); 
jQuery("#activitycategory-tabs li:odd").hide(); 
</script> <?php
} 
add_action( 'admin_footer-post.php',     'convert_root_cats_to_radio' ); 
add_action( 'admin_footer-post-new.php', 'convert_root_cats_to_radio' );

Answer 2

My Radio Buttons for Taxonomies plugin converts any taxonomy (including categories) to radio buttons so that only 1 term can ever be selected. Though I guess it doesn't handle restricting/disabling the top-level terms.