I'd take a sharp object and write the following notes on their door:

"Hi there, I was going to write this note on paper but I didn't have any on me so I figured this was the next best option. Anywho, just wanted to let you know that your car is parked on the sidewalk and blocking people from walking safely away from the street. I know it was probably an accident, so no biggie! Just wanted to let you know for next time so you can be mindful of others. Have a great day, friend!"

Bonus points - If I have them on me, bb pellets and super glue. You take the air stem cap off their tire valve, put in a drop of super glue and then a bb pellet. Screw it back on. It will slowly leak air out overtime, and the driver will go crazy trying to figure out why that tire is slowly letter air out.