Ah you are comparing it to how reddit recently filtered some subs from appearing on the front page of the internet. Yes something like that could be possible, like making a greylist of tags like #nsfw, #gore, etc. Maybe logged in users should be able to setup their list of blocked tags though.
I thought it would be an opt in for the poster, something like:
The only way to censor content in a private tag would be if all witnesses decide to not write content from a certain private tag to the block-chain. So you would need 95% of the community to think the content is obscene rather than one person with an agenda before the content is banned.
Expanding on the idea: you could allow people to register, own, moderate, and collect the curation rewards from their owned private tags.
Without a hard-fork to implement private sub-reddit style tags, I suppose an option to not to have your content show on trending, and hope agenda driven down-voters don't bother you would be nice.