Fundamentally this problem is derives from the fact that it is difficult for people to learn how to write and structure their ideas. Hell thats basically the whole point of PhD programs, and people spend years finishing them.
With the internet, it becomes really easy to just pay someone else who already knows how to write to do the work for you.
The only real solution to this that I see is to make these services legitimate as a teaching outlet rather than as a cheating outlet. Force people to write a draft. In editing, don't rewrite, just comment and give people feedback as to how they can improve the structure of their document. In general have the people feel that they are getting taught rather than just getting a product.
My hope is that creating a legitimate service will encroach the market people like this target, and will actually force students to learn something through their writing assignments.