Thanks for the review. But I have to say that turning on "Use plugin" has nothing to do. As there is no plugin for R-Antispam yet.
What you should have set is the Akismet integration with an Akismet key. That's what should have stopped the spam registration.
PS: If you want to remove the bottom back link there is an option in configuration for it.
I only wish I could rate it better than excellent.