" If you encounter an unidentified aircraft flying in commercial airspace with its transponders off, what do you do?"
What airspace class? A to G? I assume commercial airspace means outside of military restricted areas. In what national jurisdiction?
Your post doesn't look like it came from someone who'd considered the subject, and had internet access.
ATC reporting an echo, primary only, in Class G UK Airspace is not unusual. I don't think they do anything other than alert aircraft who might encounter it.
Gliders have a very low carbon footprint. Two seat gliders produce twice as much CO2 as single seaters, but no more than the occupants would be producing if seated on the ground, and much less than if they were walking.