They could do it anonymously. The difference between steemit and reddit is that on steemit, a downvote from a high value account hits the recipient's bottom line. For example, if you're trying to launch a start-up music business on the blockchain, how long can you survive high-stake downvotes from the recording industry? They don't seem to mind the bad publicity that comes from takedown notices.

I hope you're right about the ability (and willingness) for another large stakeholder to nullify abusive downvotes.