The new Freenet (formerly Locutus) is more of a communication than a storage medium, which should make it less susceptible to the kind of problems you're referring to. That said, even on Hyphanet you really need to look for something bad to find it as the default indexes are curated.

We’re also developing a decentralized reputation system based on the "web of trust" concept. This system is intended to help filter out antisocial content—like spam or worse—without relying on any central authority.