I think you could have politely pointed out that they had parked in an EV fueling station, not a regular parking space.
"ICEing" has been a long-term problem with public charging infrastructure. In California, we have some laws on the books that allow ticketing and towing of cars that are parked in spots that are denoted by signage as EV charging only. I
hope that the Supercharger locations have proper signage here in CA. We also had a long period of hybrid cars parking in the spots, apparently because they believed the "it's half electric" marketing message from Toyota and others. That seems to have died down now that plug-in cars are more common and people are starting to understand the difference.
Anyway, what I did was to print up two stacks of notes. One is directed to straight ICE cars parked in EV spaces, and the other was specifically directed at the hybrids. I kept the tone polite and included a "perhaps you didn't see the sign?" line in the one for straight ICE. But I pointed out that parking a non-EV in a spot reserved for an EV would be like parking in front of a gas pump, and could prevent somebody who really needs the car from getting home. On the back, I included a reference to the California Vehicle Code section under which they could be ticketed and towed. When I put the note under the windshield, it was the CVC section that I left exposed, hoping that might get their attention.
I have made use of these on a number of occasions. The charging infrastructure for the old RAV4-EV was so scarce that I never parked my car in one of the spots unless I actually need the charge, but I generally drove by the spots to make sure the chargers were working. If I found an ICE or hybrid in the spot, I'd leave a note.
With the J1772 infrastructure, there are many more alternatives if a spot is ICEd, but the Superchargers are rare and precious, so we probably need really good signage and vigorous enforcement.
I think Tesla has made one really good move with their station design -- it looks like a gas station, and most people should recognize it as such. It wouldn't occur to most people to park in a gas station.
Here's a screen shot of my notes. If anybody thinks they're useful, I'm happy to share the file.