No Spam by Guests!
Download this mod
This small modification makes guests and spam-bots unable to post any links on your forum.
It works like a charm on my forum, it keeps all the spam bots away
When a guest tries to post a message that contains a link, he will not be able to submit the message and gets informed that he should register, if he still wants to.
You can try posting a link as guest on my forum to see how it works.