If I put goatse in this comment, others can see it. Then mods will remove it.
If it were on IPFS, the same would happen except the old version may still be accessible if people are still distributing it.
You'd only come across it if you were deliberately looking at an older version.
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.
So maybe don't build a Hacker News clone on top of IPFS? It's not meant to be to be a solution to every problem.
This page would be an IPNS address under the admin's control which would point to some IPFS hash representing the current goatse-containing state of the page. The admin would then create a new page, which would get a new IPFS hash (since it's new content) and point the IPNS address to it.
As it turns out, you cannot ever really delete information, you can simply change where your "well known" pointers point to.
I definitely agree from a technical level, you can't ever guarantee the deletion of files on somebody else's machine.
So the question is, how do we build systems that enable users to protect their communities, without them becoming yet another tool for abuse?