like if you want to block out / censor certain words?
You'd need to go into the PHP, find the part where it processes the form input, something like this I'd imagine: (This isn't exact though)
// Where the form data is processed... (or top of page, within <? ?> block of course)
if (eregi("(>[^<]*)(word you want to block goes here)", $_POST['name of form variable here'])) die("message to display if blocked word is found");
You might even do it at the top of the page before anything else (one word of warning: formatting will be screwed up possibly, using die() because the script execution terminates after the die() command. It will cause the post to stop (as long as this is done BEFORE the data gets written into the database (look to do this before a mysql_query() command))