As a writer, I find it offensive, not to mention it's a "crime" when someone tries to "steal" someone’s intellectual property. When someone copies your material then further labels it with "their" name, it is considered "intentional" and this is illegal. Contact an intellectual property attorney as quickly as possible.

The owner of a copyright can sue in Federal court for copyright violation and the plagiarizer may end up paying the author's attorney fees and loss due to the infringement.  Further, simply because something is not registered as a copyright does not mean they can "steal" your material.  Intellectual Property Law further extends to "First Right of Use".  If an author can prove they wrote it prior, the same rules may apply in court. 

One of the ways we as authors can combat the theft of our material is to do frequent "Google" searches in quotes by simply cutting and pasting key words.  Try it.  You never know who is "stealing" your work.