That is a good attack vector, but the extent is pretty low. First off, you can't upvote for more than the upvote is worth nor more than your HP. So, this would take a scammer posting others' content, getting high rewards, + being/having access to a large stake. Most scammers don't keep HP powered up, they steal it right away, so their free upvote would be worthless.
In the rare scenario where you had a large stakeholder posting other content, they would quickly be found out, curators would stop voting them, and the downvotes would mount up as word got around. We had a similar scenario. Actually, that's what I love about Hive. It seems like we have seen it all; even the most unlikely of events take place.