Determine the type of behavior that likely indicates a server is shared with many different people, which should be extremely easy, then send that user a series of warnings culminating in banning their Plex account if they don't comply, with a transparent easy appeals process in case they get it wrong.
Alternatively: don't have Plex accounts in the first place. Distribute the software in such a way that it can be deployed silently and independently of any connection to the company that creates it. (In ancient times, this was actually the normal way of distributing software - you went to the store and bought a CD with a program on it, and then you could run that program on your hardware without ever interacting with the company that made it.)
