A few solutions:
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Federated identity services. Like old Skype had identity service as its only centralized part, one can make the same thing, but federated. Would require registration and finding one just like the comment I was answering to complains.
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F2F and trust elements - people can trust each other via QR codes or something, with 1 trusted peer beating any number of untrusted peers.
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Reputation system elements (like Kad network has), which is not enough a protection, but makes them more expensive.