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Once you know the ip address, you can block it in the Administration section under Preferences-->General Settings. There is a section there for just that purpose.
I'm not too sure that you can block certain phrases but it may be possible.
You can also block some bots from your module by adding this line to your robots.txt file in the root folder:
disallow: modules/xfguestbook.
All in all, it's a decent module; it's just being abused by a bunch of jerks and spammers who are doing nothing but confusing me. Right now, I don't know if I need to get my breasts enhanced or my private parts enlarged!