Since I got tired of deleting entries from spammers, I disabled the possibility to add new entries to my guestbook.
Entries, whose only reason obviously was to promote the own site, became more and more frequent, and I suspect that Google's PageRank, which attaches great weight to links from other pages, is to blame for that development.
If you honestly want to comment on something at my site, I'd appreciate a note via my contact form.
Thanks!