<?php
/*
Plugin Name: Prevent Category Deletion
Description: Prevent deletion of categories. Modify the $undeletable array to suit your setup. Use Category SLUGS.
Author: brasofilo
Version: 1.0
*/

add_action( 'delete_term_taxonomy', 'wpse_70758_del_tax', 10, 1 );
add_action( 'edit_term_taxonomies', 'wpse_70758_del_child_tax', 10, 1 );
add_filter( 'manage_edit-category_columns', 'wpse_70758_cat_edit_columns' );
add_filter( 'manage_category_custom_column', 'wpse_70758_cat_custom_columns', 10, 3 );

$undeletable = array( 
	'article',
	'blog-post',
	'podcast',
    'tv-show'
	);

function wpse_70758_del_tax( $tt_id )
{
    global $undeletable;
    $term = get_term_by( 'id', $tt_id, 'category' );

    if( in_array( $term->slug, $undeletable ) ) 
        wp_die( 'cant delete' );
}


function wpse_70758_del_child_tax( $arr_ids )
{
    global $undeletable;
    foreach( $arr_ids as $id )
    {
        $term   = get_term_by( 'id', $id, 'category' );
        $parent = get_term_by( 'id', $term->parent, 'category' );

        if( in_array( $parent->slug, $undeletable ) ) 
            wp_die( 'cant delete' );        
    }
}


function wpse_70758_cat_edit_columns( $columns )
{
    $columns['tt_id'] = 'ID';   
    $columns['undeletable'] = 'Undeletable';   
    return $columns;
}   


function wpse_70758_cat_custom_columns( $value, $name, $tt_id )
{
    if( 'tt_id' == $name ) 
        echo $tt_id;

    global $undeletable;
    $term = get_term_by( 'id', $tt_id, 'category' );

    if( 'undeletable' == $name && in_array( $term->slug, $undeletable ) )
        echo '<span style="color:#f00;font-size:5em;line-height:.5em">&#149;</span>';
}