Create a test user and see if the content still shows. IIRC it's to do with the recent activity being cached for the user, so it doesn't make sense updating the cache to remove it from recent activity if the user has already seen it. I believe Kier's example was if there's an error in a newspaper article - you don't ask for the newspaper back from the people who have already read the article, you just fix the error for the people who haven't read it yet.
I deleted the content as well when using the spam-cleaner.
Within "What´s New" everthing deleted fine. But within "Recent Activities" in the spammers-profile and also within "Recent Activities" (Members>Recent Activities) the spam-text is still showing up.
After using the spam-cleaner I also used the caching-tools, but it did not remove the spam from Recent-Activities.