Depending on the type of attack, this defence is either part of the protocol, or can be coordinated by users in the network. Further reading here:
Phone-a-friend consensus aka “weak subjectivity” doesn’t meaningfully differ from the administration of centralized Git repositories. If your blockchain requires human intervention to resolve disputes, as do all pure proof of stake implementations, you probably never needed a blockchain to begin with.
That’s a great question, and one Jude C. Nelson — who has a PhD in distributed systems from Princeton — is better equipped to answer [1] than me (or you, probably):
“PoW requires less proactive trust and coordination between community members than PoS -- and thus is better able to recover from both liveness and safety failures -- precisely because it both (1) provides a computational method for ranking fork quality, and (2) allows anyone to participate in producing a fork at any time. If the canonical chain is 51%-attacked, and the attack eventually subsides, then the canonical chain can eventually be re-established in-band by honest miners simply continuing to work on the non-attacker chain. In PoS, block-producers have no such protocol -- such a protocol cannot exist because to the rest of the network, it looks like the honest nodes have been slashed for being dishonest. Any recovery procedure necessarily includes block-producers having to go around and convince people out-of-band that they were totally not dishonest, and were slashed due to a "hack" (and, since there's lots of money on the line, who knows if they're being honest about this?).”
See the “spawn camping” description and defence in my prior link.