I'll usually just sort of "scoot" them aside, if I want to sit there.
I was told once by someone "hey, our chairs were there" I kindly responded "oh, nobody was sitting in them" then went on to ignore them. They got a a little pi$$ed, but who cares? I figured they'd get over it or stay mad a long time.
Once, a guy in a new pickup took up two parking places so no one would park next to him...............I did anyway. I squeezed in between the curb & the driver's door, he had to enter through the passenger's side.
I can really be a richard <gr> if I want, and you force me to.