Batch Strip HTML tags from post/page content. BACKUP YOUR DB BEFORE USING THIS! The plugin should deactivate itself if it's successful.

remove-html.php

<?php
/*
 * Plugin Name: Remove HTML
 * Description: batch Remove HTML from post_content
 * Author: Marc Gratch
 * Author URI: http://marcgratch.com
 * Version: 0.1.0
 */
function rh_get_all_ids(){
    $args = array ('post_type' => array('post','page'), 'fields' => 'ids', 'posts_per_page' => -1);
    $all_posts = new WP_Query($args);

    return $all_posts->posts;
}

function rh_strip_html_tags(){
    $all_ids = rh_get_all_ids();
    foreach($all_ids as $post_id){
		$content = get_post_field( 'post_content', $post_id, 'raw' ); 
		$content = strip_tags( $content, 'a,strong,p' ); //the second parameter are allowed tags
		$post = array(
			'ID'           => $post_id,
			'post_content' => $content,
		);
		wp_update_post( $post );

    }
    deactivate_plugins( plugin_basename( __FILE__ ) );
}

add_action( 'shutdown', 'rh_strip_html_tags' );