SOMEONE HELP!!!

I have the Advanced Guestbook 2.3.1 on my website (just like this one). I have trouble with people putting inappropriate sites in the guest book.javascript:emoticon('')
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I wanted to know how to disable a guest from entering a website URL (homepage)

I looked through General settings and couldn't find a way to disable that.

Also, I know how to block an IP but how do I bann (block) a host?

I'd appreciate any help...