In this article on SPAM control, we created a tasty honeytrap for our SPAMbot. This trap actually catches about 98% of the SPAM attempts at my site, but we can do better.
The other article used the fact that SPAMbots fill all form elements, even those that are invisible to people. Now we're using the fact that they do it too fast. A person needs time to write on the form. A bot doesn't, unless deliverately the coder introduces in the SPAMbot the ability to wait.