I am not sure if you have noticed but you seldom see trolls around steemit. The reason being that once they are identified they usually get downvoted/flagged to oblivion by the community and their posts or comments become less visible.

You also need to consider that all posts and comments are recorded in the blockchain (which no one owns). Once there, nothing can be deleted. It might not be visible in the website but you can see them using other tools or interfaces.

So once a piece of data is recorded in the blockchain it becomes inmutable and is recorded for posterity (unless if you are referring to Ethereum ;)