Don't think you could stop it. nothing stops you posting this article on other platforms. You wrote it so it's not copied. Unfortunately the greedy will do it. They will post every article they wrote for Steemit on the other platforms. Wish you could black list Steemians who operate on other competitor platforms. you know how long it has taken you to build your reputation and standing in the community. They will be trying to do it there with all the shortcuts. The genuine newbies starting on those don't stand a chance.
No, I know and I ma not sure you would want to stop it completely but, it is going to throw lots of new complications into the ecosystem considering the amount already here :)