We’re not aware of any captcha-like solutions that Nabble could enforce while posting a message. Most of us are using the mailing list directly from our inboxes, and we have a strong policy against messages coming from users that are not subscribed to the list, so we’re not seeing any of the SPAM that’s visible on the Nabble web pages. Technically they should be dealing with it on their end, but it looks like they don’t. Somehow users are registering on the online forum and bypassing their security measures.
I sent a message earlier last year about volunteers that are willing to help clean up the SPAM messages on Nabble. If anyone is interested, we can hook them up with maintainer accounts to help us remove those unwanted messages in the online forum.
If there’s any other forum/online service that can import our mailing list history and operates the same way as Nabble does, we should try it.