Interesting - I just got my first akismet caught spam post. I then deleted the spam post and the spammer, celebrated this awesoem feature. But then I went back to my topic list. The topic the spam post was on is now at the top of the list! This seems like a bug to me since I don’t see any reason to give this ancient topic that nobody else has thought to reply to zombie status.
That’s how last activity date has always worked. And the post is still there, just deleted.
I suggest having an auto-close of old topics if this is an ongoing problem, or close the ones that spammers seem to “like”.