Contact the owner of the blog first, then their hosting service provider about this issue. I'd provide screen shots of your poem and send it in to the service provider to give them proof that you're the original owner of the content. It'd be best if you have a date that shows when you've written the poem.
You can find out someone's web hosting service provider by going on whois.sc
Use Copyscape banners to warn users not to steal your content. Also Google alert to alert you whenever your keywords pop up on the search engine.