Use a thinking activity to block bots (Score 1)

So, the problem definition is: we want any live human to be able to post a comment, but we wish to disallow bots from doing the same. How to do this? Several ideas come to mind, all involving easy activities for a human but difficult for a bot. For example, when someone tries to post a comment, randomly select and display a small image of an object in a thumbnail view and then have a multple choice popup menu of words for that item. Like "dog", "shoe", "lamp", etc. The name of the item must match the picture in order for the post to succeed. Throttle failure retries to like one a minute per IP address to avoid a bot from trying all selections. Also reselect and display a different image and word list on failed attempts. With a list that has 10 items, there would be a 10% success rate for a bot that randomly guessed menu items. That would mean a 90% failure rate. Have the site record and track if a particular IP address has a high failure rate. Then add to a spamlist on a server somewhere that other sites could share. I guess-bot would quickly blacklist itself.