There is no stopping it. You cannot police it away.
Every single social media site on the internet encourages its users to share interesting content, people will do the same thing here.
What steemit needs to do is embrace it and encourage it like they do with youtube videos. Make it easier to embed the content than it is to copy it, and this place will explode with cool content.
There is an open source project called oembed that is designed to share content ethically, by embedding the content and providing a link to the author. It has gone mainstream now, there is even an enterprise level service based on it called embed.ly that provides the service for a monthly fee. I wrote up a more detailed description here:
A solution to copied content that would make steemit explode with cool posts