Interesting problem. The perpetrator may be using software that finds your blog and feeds it to the blogspot website. There must be a way to prevent this, but most bloggers actually welcome it. Except that the parasite should not publish your whole articles. They should publish just about 500 words or so with a link back to your website.
You've done the right thing with the Yoast plugin, sending a post back to your blog, maybe. I don't know for sure, but too many links coming from their blog to your blog may hurt you. It shouldn't. But we are never sure how the search engines operate.
You may also think about putting an author's box close to the end of each post. The author's box would say who wrote the article and subtly say the name and URL of your website. E.g. "Shawa lives in .... and is the owner of xxxxx.com. This article, about . . . . etc." If you place a hyperlink to your post in the author's box, you might want to disable the link in Yoast.
Best of luck. Let us know if you come across a great solution.