Turn bike upside down. Shield everything with bits of scrap steel, hit the aluminum kickstand piece underneath that the flat socket hex head screw is threaded into with the hottest torch you have, whack either the exposed end of the screw or the aluminum with a hammer, or whack the head with a punch and hammer, try again. You want to shock the steel fastener and loosen the aluminum oxide that is gluing it to the crappy alloy the kickstand is made of.