Captcha for Conversations?

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Is there a way to enable Captchas for comment submissions? I have a captcha on my current website comment system and I still get occasional spam comments, so running comments without any control is completely unthinkable.

Looking a bit through the 5.7 code the only place where I could find the captcha being used is the new user registration, but people should be able to comment without registering.

Thanks,

Stefan