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As of... last year, I think it was hardfork 20, a user can edit their post on steem as often as they like, and in practice that means the owner of the post can delete the contents of the post. Likewise a frontend can opt to not show a users posts. However, neither of these actions remove the post from the chain.

But editing posts does not delete the old posts. The front end show the edited post, but the old one is still on chain you just need to find it. (For example on steemd)

Nothing can be removed from the Steem blockchain by anyone. The reality is that it is very difficult to prevent anyone from plagiarizing your content if you make it public. That is one of the reasons why with OpenSeed we are putting a lot of thought into how we enable creators to control who gains access to their content, and providing those creators with the ability to charge people for that access. Your public content should function more like an advertisement for your private content which only a privileged few have access to. This also makes it easier for users, communities, and creators to police plagiarism.

"Nothing can be removed from the Steem blockchain by anyone."

Without a hardfork. Any change whatsoever can be undertaken by consensus of 17 of the top 20 witnesses. It may be extremely unlikely, but not impossible.