I think it's already prevented. No high steem power will vote such a post when the exchange is a vote from lower steem power. Also it applies on high to high steem power upvotes exchange. Why would people with decent earning will exchange upvotes that will harm their status. No one wants to be banned when they can earn it without violating rules. There's no such method or way on cheating here. Well that's my point of view. Everyone has their own opinion. =)