(Note: The advice below should not be considered legal advice. If you believe a theft has occurred, you should consider retaining the services of an attorney that specializes in copyright law.)

• First, check the content on Google, or use a plagiarism checker such as CopyScape to make sure the client is actually using your work. Also, double-check to make sure you were not paid within your Zerys account.
• Start a conversation with the client through their Zerys profile first. If they're unresponsive, you can attempt to contact them via the contact information on the site where your content has been published. To start with, we have found it's always best to assume there has been a misunderstanding, and to remain polite and professional. Consider asking asking them to explain to you why they have published your work without paying for it, and give them options to rectify the situation. You can choose which options you want to give them. These options can include removing the content entirely from their site, compensating you for the content, or crediting you as the author on their website.
• If the client does not respond and agree to rectify the unauthorized use of your work, you can send a Cease and Desist message to them, which is basically a formal letter stating that they must rectify it or further action will be taken. Templates of Cease and Desist letters can be found online.
• If, after a reasonable amount of time, actions have not been taken to make proper amends, consider contacting the client's supervisor, web host, and/or ISP. If the content is hosted by a 3rd party website, you can contact that website and give them the URL of the content you are claiming was stolen, and explain that you own this content and that it was published without your permission, and request that they take it down.
• Consider contacting search engines to request that they take down your copyrighted material. You can learn more about the Digital Millenium Copyright Act and get instructions for filing requests at Google and Yahoo!
• If all else fails, consider hiring a copyright attorney

What NOT to do:

• Try not to lose your cool. Remain polite and professional and assume this is all the result of a mistake. If this doesn't work, you can rachet things up later, or ultimately let your attorney do it for you.
• Don't resort to posting negative things about the client on their website, on social media pages, or anywhere else out on the web. Even if you're in the right, this can be used against you by the client later and could end up causing you headaches in the future.
• Don't delete anything. Save records of your work, its misuse, and all communication with the client, with screenshots and timestamps if possible.