I'm just another WordPress.com user, not affiliated with WordPress.
I've had some experience with your issue in the past. The truth is there is no technical ability for WordPress to block any party from following your blog. WordPress isn't Facebook and doesn't have that Facebook functionality.
For that party following and plagiarising your stuff, the only workable way is to report that party to its webspace/blogspace provider. If that party is on a self-hosted site, then the next course of action is to report it to its ISP provider.
Other than that, there really aren't many countermeasures available.
Hope this helps.