That's just one of Instagram's many growing problems. There are other social photo networks and many of them don't have the problem of people snagging and reposting others' works. Unfortunately, Instagram captured "the magic" and has far more users than its competitors. I was on Shnap for a while. I like a lot of the controls the app and network has. But I rarely post to social photo for the time being.
There should be a way to report copyright violation. As an artist, if my work was pirated and then passed off as someone else's I'd be really pissed. There probably is a way to report flagrant abusers of copyright violation to Instagram, but I suspect it's not easy to do within the app and has to be done through the website.
Best thing to do is to unfollow anyone who simply posts others' work.
It's unfortunate, but if you want to play in Instagram's sandbox, you have to maneuver through a lot of crap to get to the good stuff.
=M=