A cease and desist letter generally doesn't have to be a physical letter. Nothing stopping a law firm from DMing you a C&D. If an artist ignores a DM'd C&D, a company could start proceedings in court and as part of that order the hosting site and other sites to turn over account info that can be used to ID someone. Very expensive and time consuming, but often companies only need to go after a few high profile cases to scare everyone else doing it.