The Steem blockchain is decentralised, so no content can be blocked there. But Steemit.com the website is centralised, so the admins could block at that level if required. The offending content would merely be filtered out, the same way as much content is filtered/removed from the internet today.
Google, for example, automatically indexes all content, but is frequently asked to remove some content. Similar with Steem and Steemit. The Steem blockchain would contain all content, but Steemit would not index some of it.
The dodgy parties could still use the blockchain for storage if they wanted I suppose and nothing could be done about it as it's decentralised, but yeah, the website would be safe as the offending content would simply be filtered.
...would not present some of it - I think that's more accurate.