We can't. Removing that kind of content from the Steem blockchain would mean censoring it which, even if it's for a good reason, is something Steem is designed to not let happen. You are right, content can't be censored and I believe it's better that way. Of course, it has its downsides (child pornography, murder videos, etc...) but ultimately it's not the Steem blockchain that has to deal with them but the websites that interact with it. I saw in the comments that witnesses could edit the blockchain by consensus, if that's true, I don't like it. It's an open door to censorship.