Hello,
Search is your friend. This site is where he got the hack. Most of the hacks for vBulletin are found on this site.
You need this:
First Page of Thread
Read down a few posts in that thread and it tells you how to make it limit to one post. After the one post, a phrase will be displayed. Edit the phrase to say whtever you like. (Example: You need to register in order to read the replies to this question.)