Lots of SPAM answers

We are getting a ton of SPAM on our qa forum as answers. I assume that most of our spam filters only work on comments, and that is why the spam answers keep appearing.

But I'm also seeing most of the spam is from user id 0, a non-user. Our forum is configured to prompt non-users to register before posting, so I don't understand how this is possible.

Just after posting this, I noticed that under Accessibility settings, visitors were allowed to "Add answers". I assumed this would also prompt them to register, but if that's not the case, then maybe this is the explanation for the problem.