<script type="text/javascript">  
var messages = document.getElementsByTagName('td'), j = messages.length;  
while (j--) if (messages[j].className == 'posttdMessage') {  
messages[j].innerHTML = messages[j].innerHTML.replace(/(ваши|матные|слова)/g, '<span title="$1" style="color:red;">***</span>')  
}  
</script>