I feel this could be solved by crowdsourcing, assuming the support from the major phone manufacturers and possibly governments.

- After a call from an unknown number concludes, the phone could offer an option to the user to mark it as "spam" (blocking the number in the history would have the same effect).

- The spam mark would then go into the central database. If a phone number was marked as "spam" too many times, it would no longer be able to ring the phone (maybe it gets diverted into the "spam folder", like e-mails).

Or am I being too naive here? What are the obstacles for something like this actually happening?