There's no way for WordPress -- and, therefore, no way for s2Member either -- to know who each visitor is unless s/he logs in. So the short answer to your question is that s2Member can't do what you want.
You can Google for examples of how to do this. There's also a WP plugin in the repo called Leaky Paywall which might do what you want (I've never tried it), and there might be other plugins around with similar functionality too. Good luck!