IF you are the copyright holder, filing a DMCA take-down notice is fairly straightforward.

If you are not, there are actually really GOOD reasons why you can't do that, legally.

Say I write on my own site, using my own name, and I write on another site using a pseudonym. I post a story that is substantially the same, both places. It is not your place to assert that one or the other is violating copyright, though as a writer, I'd very much APPRECIATE your caring enough to check.

The DMCA take-down notice includes your legal affadavit that YOU own the rights to the infringing material. This protects the copyright holder, as well - you may feel you'd be doing them a favor, but more likely, others would use DMCA notices in a trollish, abusive manner to silence people they didn't like, and sites like Medium should not have to play peacemaker.

BUT - there should be a better way to report OBVIOUSLY infringing material - and to explain why it's CLEARLY problematic, so that Mediium could, at least, remove such postings from the PayWall.

Until then, you can make a good-faith effort to notify the real copyright holder (beware, though, that you are not notifying a plagiarist!) and let them file the DMCA take-down notice. Help them out, if they don't know how to do that.