Why shouldn't that be handled at the application level? Just like with git, if there's content you don't want anymore, you orphan the block hashes in whatever structure the application uses to store and display content; IPFS nodes could still store the abusive comments or media, but nobody would find it unless they were looking for it or randomly fetching blocks.
If the ipfs-based application doesn't have some means (by a group of administrators, or by community consensus) to orphan (de-link) user-contributed content that's abusive, then that app needs to be improved. It doesn't seem like an IPFS-protocol-layer problem.
You could add another layer on top of IPFS, something like IPFS-O (ownership) or IPFS-C (censorship), which used either a separate DHT or a centralized service to allow people to register never-before-seen block hashes by generating a keypair and uploading a signature of the block hash. If the content later needed to be removed, the signature could include contact information, and the signer could be appealed to (or sent a court order) to sign a removal message. It wouldn't really work, though. Nobody would run ipfs nodes paying attention to such a meta-service. And if the service were centralized, there would be grave concern over censorship by whatever entity ran it. And people could abuse the service by registering data blocks that haven't shown up in IPFS yet, claiming ownership when they don't really own it. You'd have to go to the courts to resolve that, the courts could issue an order to a centralized operator, if it's centralized, but again, nobody would run IPFS nodes respecting such a centralized censorship-enabling service, so the whole thing would be futile. And if the censorship service were decentralized, it could be sabotaged by enough libertarian nodes refusing to store or pass along removal messages.
If the ipfs-based application doesn't have some means (by a group of administrators, or by community consensus) to orphan (de-link) user-contributed content that's abusive, then that app needs to be improved. It doesn't seem like an IPFS-protocol-layer problem.
You could add another layer on top of IPFS, something like IPFS-O (ownership) or IPFS-C (censorship), which used either a separate DHT or a centralized service to allow people to register never-before-seen block hashes by generating a keypair and uploading a signature of the block hash. If the content later needed to be removed, the signature could include contact information, and the signer could be appealed to (or sent a court order) to sign a removal message. It wouldn't really work, though. Nobody would run ipfs nodes paying attention to such a meta-service. And if the service were centralized, there would be grave concern over censorship by whatever entity ran it. And people could abuse the service by registering data blocks that haven't shown up in IPFS yet, claiming ownership when they don't really own it. You'd have to go to the courts to resolve that, the courts could issue an order to a centralized operator, if it's centralized, but again, nobody would run IPFS nodes respecting such a centralized censorship-enabling service, so the whole thing would be futile. And if the censorship service were decentralized, it could be sabotaged by enough libertarian nodes refusing to store or pass along removal messages.