In the last several months, my personal website was hit by spammers everyday (in guestbook section). It's because the hosting services disabled the GD library and it will be enabled again if we upgraded the basic package to professional package. I used GD to make small (and simple) CAPTCHA code to prevent bots to fulfill my guestbook. Since then, i make a simple validation based on blacklisted IPs, but it wasn't effective as well, since they can use open proxies and they can do IP Spoofing using tools that are available on the Internet.
Last month, my account on the hosting services was expired and then we changed to my friend's hosting service and i asked him about the GD status and he said that it is enabled, so yesterday, i re-upload the code to enable CAPTCHA code in my guestbook to reduce spammers. I know somehow, they can broke it, but at least, it has been drastically reduced for now and i'm pleased with that, since i made the code by myself (with some help of PHP Manuals). I know that there is a PEAR extension for CAPCTHA, but i don't know whether the PEAR is installed or not, so for safety, i just create some simple code but it worked (at least for my case). I will also find better code with simpler technology to be implemented soon.
Next step is to implement the same code in my writings where visitors can give their comments on my articles that i wrote.