WordPress : supprimer totalement les tags. Détails et explications : http://www.diije.fr/wordpress-supprimer-tags/

functions.php

<?php
/* suppression de la metabox des tags et des formats sur la page d'ajout/edition de posts */
function dfr_remove_tags_metabox() {
    remove_meta_box('tagsdiv-post_tag', 'post', 'side');
}
add_action('admin_menu', 'dfr_remove_tags_metabox');

/* suppression de la colonne "Mots-clefs" sur la liste des articles */
function dfr_remove_tags_column($defaults) {
    unset($defaults['tags']);
    return $defaults;
}
add_filter( 'manage_posts_columns', 'dfr_remove_tags_column');

/* suppression du menu "Mots-clefs" */
function dfr_remove_tags_menu() {
    global $submenu;
    unset($submenu['edit.php'][16]);
}
add_action('admin_head', 'dfr_remove_tags_menu');
?>

SQL.sql

DELETE a,b,c
FROM prefix_terms AS a
LEFT JOIN prefix_term_taxonomy AS c ON a.term_id = c.term_id
LEFT JOIN prefix_term_relationships AS b ON b.term_taxonomy_id = c.term_taxonomy_id
WHERE c.taxonomy = 'post_tag'