What an absolute piece of shit! However, I wonder if you could go directly to the blogs and websites that your work was sold to and let them know that you were the author of the articles (you have proof, right?). Say that you are no longer writing for the guy but if they like your style, you would be happy make an arrangement directly with them.
I would also write an honest review of the guy and mention that you found your work elsewhere and that you found him exploitative and abusive.
Personally I am appalled that Upwork supports this kind of exploitation. They can lay basic standards and put a stop to this kind of behaviour.