Reference : http://askmichel.icoder.com
When you have a Contact-Form on your site, you are having problems with spam submissions to the form. Unfortunately, spammers have programs that find forms on the web, and automatically fill them out with spam messages.
The standard way out is the use of captchas. However, there are bots, that can recognize simple captchas in a second. Let’s review ideas of Form-Spam bots blocking without captchas.