I don’t think this can solved legally without compromising anonymity. You can block unrecognized clients and punish the owners of clients that behave badly, but

I don’t think this can solved legally without compromising anonymity. You can block unrecognized clients and punish the owners of clients that behave badly, but then, for example, an oppressive government can (physically) take over a subversive website and punish everyone who accesses it. Maybe pseudo-anonymity and “punishment” via reputation could work. Then an oppressive government with access to a subversive website (ignoring bad security, coordination with other hijacked sites, etc.) can only poison its clients’ reputations, and (if reputation is tied to sites, who have their own reputations) only temporarily.