Spam guestbook entries before the panel 
Author: Mike P.
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Why not set the guestbook up to reject any post where the name is a continuous string. And create black list and a wqhite list. Anything on the blacklist would never reach the control panel. Anything which failed the continuous string would go to a second control panel to be banned or accepted
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Better yet set the guest book to refuse to send any entry with html or php code in it to the control panel.
I believe it would be very easy to block anything with the sub string "href" .
That is the machine or moron which is trying to put malicious links on the website would be rejected when they click submit if they have the substring "href" anywhere in their post. Of course any other html or php string which could be used should also be detected by the blocking code.
I even think you could send all such posts automatically to any authority which can police the web.