I forgot to mention that the most effective way for many of us to cut spam down was to enable passcode field for the registration, and show the passcode for humans. In recent tiki versions there is a checkbox for that. In older tiki versions, you can do that by hand using jquery code: see this tutorial: https://doc.tiki.org/Anti-spam
Tom, I wish you commit your code to Trunk for others to re-use it, etc.
Have you read this?: