I’m developing a simple peer-to-peer app in .Net which should enable users to share specific content (text and picture files). As I've learned with my last question, inappropriate content can “relatively” easily be identified / controlled in a centralized environment. But what about a peer-to-peer network, what are the best methods to protect a decentralized system from unwanted (illegal) content?

At the moment I only see the following two methods:

1. A protocol (a set of rules) defines what kind of data (e.g. only .txt and jpg-files, not bigger than 20KB etc.) can be shared over the p2p-network and all clients (peers) must implement this protocol. If a peer doesn’t, it gets blocked by other peers. Pro: easy to implement. Con: It’s not possible to define the perfect protocol (I think eMail-Spam filters have the same problem)

2. Some kind of rating/reputation system must be implemented (similar to stackoverflow), so “bad guys” and inappropriate content can be identified / blocked by other users. Pro: Would be very accurate. Con: Would be slow and in my view technically very hard to implement.

Are there other/better solutions? Any answer or comment is highly appreciated.