I find that almost no spam-bots are the slightest bit “clever.” The malcontents that run them usually get a scoop of URLs, say from Google, and start slamming random data (“prescription drugs without a prescription, anyone?”) into them. They really aren’t out to trash your website out of the hundreds or thousands of URLs that might be on their trash-list.
So... put almost anything on the site which may require any sort of specific human input. (Put the instructions somewhere besides a “label” that is adjacent to the field in question.) The bots will consistently fail that test, and the bot-owners (who are busy burning in hell...) will never actually bother to find out why.