If you get the following message...
You cannot sign this guestbook at the moment!
... please read this post carefully!
This post will shortly describe the problem and solution to hopefully avoid another dozen threads about this exact message

So, if you get the above error message this means you did something to upset the JunkMark(tm) filter. JunkMark is an ANTI-SPAM filter built into GBook script.
Once you get that message your are being blocked by GBook within the same browser session. This means you have to close all your browser windows before you will be able to post to your GBook again.
To avoid getting this error make sure you DON'T enter any URLs in the "Comments" field and don't enter any common SPAM words like "Viagra", "Casino", etc...
As of GBook version 1.41 posting URL's to guestbook is disabled by default. You can enable posting URL simply by setting $settings['use_url'] to 1 in settings.php (version 1.41). This will add the "Your website" field to your GBook but you still won't be able to post URLs in comments because they do not belong there and posting URLs in comments is being abused by spammers.
The philosophy behind this is that a guestbook is meant for posting comments about YOUR website and not advertising (posting links to) OTHER websites.
Hope this helps.
Enjoy!