Because of the way G+ handles people who are not in circles, there's actually no need to filter; there's already a default filter running all the time. If you add someone, and they don't add you to a circle, your comments are only visible via the "Incoming" section of the stream. Your posts are effectively invisible to them until they view that section. The result is much closer to Twitter/broadcasting than FB.
I'm not sure how G+ handles @ notifications from people outside circles. If those are blocked as well, that's a problem.