Send a cease and desist letter to the other party and, if your artwork is part of a registered trademark, to the grantor (state or federal) of the protection with a request to terminate the mark's registration. If you haven't already, register the artwork with the U.S. Copyright Office. The other party may be willing to stop using your artwork or may purchase/license it. But be prepared to go to court.
Answered on Jun 29th, 2015 at 3:02 AM