The problem is that decentralized services only work when everyone is on decent behavior. When the degenerates onboard, you need the harsh hand of a centralized authority.
> you need the harsh hand of a centralized authority.
What you actually need is reputation information, which you can make your own decisions based on. Unfortunately the cheapest way to gather reputation is with something like a panopticon, where one party sees all interactions and everyone trusts their judgement.
It's at least theoretically possible, though, for individual nodes to securely gossip reputation information to each other, which is the model that the Matrix.org team are working towards:
