If it was me and I was waiting for my car to charge anyway, I might strike up a friendly conversation as a fellow EV driver, ask them about their car, how they like it, where do they usually charge, etc. If they were trying to figure out if they could use the supercharger, I'd explain that and offer to help find a nearby one they can use. If they weren't being friendly, I'd leave them alone. No need to try to kick them out if the others are all free, but they may be happy to chat or be looking for help.