There's a new tool available to help fight spam in the Drupal world. Spamicide is a new module that works to prevent spam submission (comments, contacts, bogus users, etc...) to your site. It doesn't require user interaction as it adds a field to Drupal forms (with a meaningful description shown to screen reader for accessibility) that is hidden by a very small css file of the same name as the field. When a bot fills in the field the form is rejected without any notice and, if logging is enabled, logged with the ip address of the offender. If the logs show that the bots have learned our little trick, the admin can change the name of the field/css file to shut them out again.
Version 5 is ready, working on 6, should be available very soon.
Feedback is welcome/encouraged.