Sony has researched and filed a patent for new technology that could be implemented to block used games.
As spotted by a NeoGAF user, patent application No. 20130007892 for an "Electronic Content Processing System" and "Use Apparatus" would associate individual game discs with matched user accounts.
Games that are discovered to be tied to another user's account could potentially be rejected. The system works, according to the document, by attaching contactless RF "tags" to each game that can be be read without a network connection.