<?php
/*
Plugin Name: Strip non-allowed tags
Description: Strip prohibited HTML tags from post text (instead of leaving them as entity-encoded text)
Author URI: http://samm.dreamhosters.com/bbpress/plugins/ 
Version: 0.1
Tested up to: 0.81
*/

function strip_nonallowed_html_tags ($s) {
	$allowed_tags = array_keys( bb_allowed_tags() );
	$allowed_tags = '<'.implode('><', $allowed_tags).'>';
	$text = strip_tags( $s, $allowed_tags );
	return $text;
}

// Run before bbPress's default filter `encode_bad` (10)
add_filter('pre_post', 'strip_nonallowed_html_tags', 9);
// You could remove `encode_bad` because KSES strips unallowed tags anyway (?)

?>