I just destroyed a spam user, and noticed that one of their posts, which is to this question:
Is the seventh spam answer to be deleted. I looked at the question and noticed it wasn't automatically protected by Community♦ already, so I've gone ahead to protect it myself.
Why wasn't this question automatically protected? According to this feature request, a question is auto-protected after three answers from new users are deleted. Did the accounts responsible for the other spam answers somehow escape the low-rep threshold?