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As Zen said, the only thing you can really do is to contact the magazines printing your poem to let them know the author credited with the story have plagiarised you. But, yeah, proving you're the original author might be difficult unless the story has been posted somewhere prior with a date and time stamp or you have the original submission email still lying around with the story attached.
I will say, though, that you have been extremely unlucky. Plagiarism really doesn't seem to be that big of a problem as far as I'm aware of. Mostly, I guess, because the people doing it get caught sooner or later, and then publishers realize they have done it before, because of course they have. If you know how to write, you're not going to copy other people's work.
I've heard about it happening with poetry once or twice before, but I've never heard about even short stories being copied like that.