It helps if you have pre-registered or registered your creative work. If you are not yet registered, it will help if you can prove the work is uniquely yours. If it is a commercial piece or a high-value item, be prepared to show how infringement may have caused damage to your company and/or reputation.
To stop copyright infringement:
Just ask. The first step is to ask them to stop using your creative work. They may not understand copyright law, or they likely thought you wouldn't notice or that "everyone is doing it" so they didn't see the harm. If someone mimicked your artwork on their website or copied your article, they will often take it down voluntarily.
Send a Cease and Desist Letter. A Cease and Desist Letter formally informs the infringer that they must stop using your creative work or you'll take legal action. You can make this letter using Rocket Lawyer or with the help of your lawyer. Include with the letter documentation that shows you own the copyright. Also, keep a copy of all correspondence for your records.
Hire a copyright lawyer. If the infringer doesn't respond, you may have to hire a lawyer to help you navigate the legal process and seek damages. It will help your case if you keep meticulous records and can prove that you own the rights to the creative works and that you have suffered damages.