BERLIN — Left-wing groups and Berlin residents prevented more than 500 far-right extremists from marching Saturday to the Berlin prison where high-ranking Nazi official Rudolf Hess died decades ago. Police in riot gear kept the neo-Nazis and an estimated 1,000 counter-protesters apart as the two sides staged competing rallies in the German capital’s western district of Spandau. Far-right protesters had planned to march to the site of the former Spandau prison, where Hess hanged himself in 1987, but were forced to turn back because of a blockade by counter-protesters.