First, sorry you are experiencing this. This is for off-platform plagiarism, meaning that you have found your story copied on a website outside of Medium. If the plagiarism is on Medium itself, please let us know at yourfriends.medium.com.
Unfortunately, there isn't anything Medium can do to prevent people from copy and pasting, and in essence "stealing", the text from the Medium post page.
As good citizens of the internet, Medium completely honors the DMCA, and all takedowns we receive. So when we are alerted to copyright infringement that occurs on Medium, we remove it until the matter can be resolved legally. We offer a public form to initiate this process.
If the site that has taken your work does not appear to have that in place, nor any contact information even, and embraces anonymity above all else.
What can you do?
As the copyright owner, you need to make a claim against them for copying your work. As there in no contact information on the site, you can do a Whois lookup to find any other information on the site: https://www.whois.com
You can also find their host:
We appreciate this is a frustrating experience, and hope this guidance helps.