Editors tend to copy and paste from MS Word a lot.

cleanup_for_writers.php

<?php

add_filter('content_save_pre', 'cleanup_for_writers');
function cleanup_for_writers($content)
    {
        remove_filter('content_save_pre', array($this, 'cleanup_annoying_writers'));

        $content =trim(stripslashes($content));

        //No JS
        if (strpos($content,'</script>') !== false) {
            $content = preg_replace('/<\/script>/', "\n\n", $content);
            $content = preg_replace('/<script[^>]*>/', "\n\n", $content);
        }

        //No CSS
        if (strpos($content,'</style>') !== false) {
            $content = preg_replace('/<\/style>/', "\n\n", $content);
            $content = preg_replace('/<style[^>]*>/', "\n\n", $content);
        }

        //No Divs
        if (strpos($content,'</div>') !== false) {
            $content = preg_replace('/[\s]*<\/div>[\s]*/', "\n\n", $content);
            $content = preg_replace('/[\s]*<div[^>]*>[\s]*/', "\n\n", $content);
        }

        //No paragraph tags
        if (strpos($content,'</p>') !== false) {
            $content = preg_replace('/[\s]*<\/p>[\s]*/', "\n\n", $content);
            $content = preg_replace('/[\s]*<p[^>]*>[\s]*/', "\n\n", $content);
        }

        $content =trim($content);

        // No Spans
        if (strpos($content,'</span>') !== false) {
            $content = preg_replace('/<\/span>/', "", $content);
            $content = preg_replace('/<span[^>]*>/', "", $content);
        }

        //No Inline CSS
        if (strpos($content,'style=') !== false) {
            $content = preg_replace('/style="[^"]+"/', "", $content);
            $content = preg_replace("/style='[^']+'/", "", $content);
        }

        //except counter-resets
        if (strpos($content,'start=') !== false) {
            $content = preg_replace('/start="([^"]+)"/', 'start="${1}" style="counter-reset:headers ${1};"', $content);
            $content = preg_replace("/start='([^']+)'/", 'start="${1}" style="counter-reset:headers ${1};"', $content);
        }

        //No Center tags allowed
        if (strpos($content,'<center>') !== false) {
            $content = str_replace('</center>', '', $content);
            $content = str_replace('<center>', '', $content);
        }

        //em not i.
        if (strpos($content,'<i>') !== false) { //change <i> tags to <em>
            $content = str_replace('<i>', '<em>', $content);
            $content = str_replace('</i>', '</em>', $content);
        }

        //strong not bolds
        if (strpos($content,'<b>') !== false) { //change <i> tags to <em>
            $content = str_replace('<b>', '<strong>', $content);
            $content = str_replace('</b>', '</strong>', $content);
        }

        //Remove character non-breaking spaces
        if(strpos($content,' ') !== false) //remove all non-breaking spaces from word.
            $content = str_replace(' ', ' ', $content);

        //Remove entity non-breaking spaces
        if(strpos($content,'&nbsp;') !== false) //no entity non-breaking spaces either.
            $content = str_replace('&nbsp;', ' ', $content);

        //Move spaces outside A tags and Em Tags
        //If there is a word character or space after, move it outside.
        //If there is not, delete it.
        if (strpos($content,' </a>') !== false) {
            $content = preg_replace('/[\s]+<\/a>(?=[a-zA-Z0-9 ])/', '</a> ', $content);
            $content = preg_replace('/[\s]+<\/a>/', '</a>', $content);
        }

        if (strpos($content,' </em>') !== false) {
            $content = preg_replace('/[\s]+<\/em>(?=[a-zA-Z0-9 ])/', '</em> ', $content);
            $content = preg_replace('/[\s]+<\/em>/', '</em>', $content);
            $content =trim($content);
        }

        $content = preg_replace('/ ([\.\!\?\,])/', '${1}', $content);

        $content =trim($content);
        //No More more tags
        if(strpos($content,'<!--more-->') !== false) // get rid of more tags
            $content = str_replace('<!--more-->', '', $content);

        //no entity &'s in text
        if(strpos($content,'&amp;') !== false) // get rid of double spaces
            $content = str_replace(' &amp; ', ' & ', $content);

        //No double spaces
        if(strpos($content,'  ') !== false) // get rid of multiple spaces
            $content = preg_replace('!  +!', ' ', $content);

        //remove extra spaces between tags and empty tags
        //back to back a tags are allowed
        if(strpos($content,'> <') !== false)
            $content = preg_replace('/!> +<(?!a)/', '><', $content);

        if (strpos($content,'></') !== false) { //get rid of common empty tags
            $content = str_replace('<blockquote></blockquote>', '', $content);
            $content = str_replace('<p></p>', '', $content);
            $content = str_replace('<div></div>', '', $content);
            $content = str_replace('<span></span>', '', $content);
            $content = str_replace('<em></em>', '', $content);
            $content = str_replace('<strong></strong>', '', $content);
        }

        if (strpos($content,'<br') !== false) { // remove empty p tags with lots of white space
            $content = preg_replace('/<br[\s]*\/?>[\s]*/', "\n", $content);
        }

        //No fancy quotes.
        $content = str_replace('“', '"', $content);
        $content = str_replace('”', '"', $content);
        $content = str_replace('’', "'", $content);
        $content = str_replace('‘', "'", $content);
    /*
        //If you really can't trust the morons to do anything right:
        //Load it up to make sure it's all formatted well, then spit it out.
        @$dom->loadHTML('<?xml encoding="UTF-8">' . $content );
        $content = $dom->saveHTML($dom->getElementsByTagName('body')->item(0));
        $content = trim(trim(str_replace(array('&#13;','<body>', '</body>'), array("\n",''), $content)));
    */
        //remove windows newlines
        if (strpos($content,"\r\n") !== false) {
            $content = str_replace("\r\n", "\n", $content);
        }

        //remove windows newlines
        if (strpos($content,"\r") !== false) {
            $content = str_replace("\r", "\n", $content);
        }

        if (strpos($content,"\n\n\n") !== false) { //remove large blocks of new lines
            $content = preg_replace('!\n\n\n+!', "\n\n", $content);
        }

        //$content = mb_convert_encoding($content, "UTF-8");

        //trim it all one last time.
        $content =trim($content);

        return $content;
    }
?>