We just had a discussion about this at our freelancer meetup last night. We've got a PM who's going to give a presentation on contracts and scoping work in January, and we were chatting about problems freelancers face. A couple guys in our group have been getting abused in much the same way you describe, although f2f and in the same town, not remote via odesk. And... trying to get people to go through a full 'statement of work' process before getting started is just something that will fall flat with most projects under a certain amount, however informal you try to make the process.
What we all agreed on is that there are some number of abusive people out there, much like you described. No amount of process, formalizing agreements, etc will save you from dealing with them. They have to be avoided, but you have to know how to identify them first, and you have to be willing to walk away from work, sometimes even in the middle of a projects. None of that is easy for some people - especially younger folks.
And yet... by the same token, I get contacted from people who tell me "my last guy just quit - just quit responding to emails - he flaked out - etc". I used to think those people were just irresponsible idiots, and I was going to provide great customer care, etc. I now realize there's 2 sides to the story, and sometimes the original party 'flaked out' because the employer in question quit paying or bounced checks, or is abusive, or a micromanager who's never satisfied and wants an ebay clone for $25 and will call you at 1am because they need help setting up their wife's email on her new iPhone.