There is currently a forum thread that discusses plagiarism. On two occasions, I wrote two different articles for two different people . I was given the job in freelancer.com. I was not paid and I left scamsters alone after some pursuing. After a decent interval, I found the articles in Hubpages in two different names. One a gentleman (!) and the other a lady(!!) When I confronted them, they simply kept quiet. I threatened them with action. No dice!
I tried to raise alarm with Hubpages management. I might as well have dropped a stone in the Pacific and expect it to bounce back.
If you write them and say "Hey! I wrote that article under contract and didn't get paid for it" you will likely receive no response. HP has neither the responsibility nor authority to enforce private contracts.