2. original CMSimple Guestbook with CAPTCHA function
Here, I present a changed version of the original guestbook containing a randomizer to function as a simple captcha. So, if you use already CMSimple 2.6, which has several functions being outsourced one of which is the guestbook function, you just replace the original guestbook (gbook.php) in the "cmsimple" subfolder with the changed version. It is driven by CMSimple like the old one. It will work (read and write) like the old one - just with the additional requirement of adding a captcha number into an input field.
To make it clear to the visitor what to do, you should also open your cmsimple language file in the "cmsimple/languages" subfolder. Here, you change the text string of the "$tx['error']['mustwritemes']" variable to something which tells the visitor what he should to with the captcha number. Of course, you can also do this in the language settings of CMSimple's configurations.
The captcha function comes with two new CSS classes (captcha, captchainput) to make it easier to format the output.
I know, this simple captcha function will not be a 100% protection from receiving spam. But it will help a lot and will make automated spam distribution more difficult.