Let's say they thought about doing it. Voldemort wouldn't even give people a chance to stun him. And most importantly he always stays in the company of his death eaters who wouldn't harm a hair of his (though he hasn't got one) And all his prisoners don't even have a wand. Roughly, the only persons who can have a talk with Voldemort with a wand in possession, apart from his death eaters are Harry and Dumbledore. Now Dumbledore uses spells only in defense in a duel with Voldemort. He never attacked him. So that leaves Harry. Harry is no prodigy in transfiguration, he has no feats of even doing something else than a small object. Spells are like chakra attacks, they can improve with time and knowledge. Also Voldy would be standing there ready to kill anyone within a second. The only spell that come into his mind at thatbelive that theyd even think about it. they had SO many hacks, and yet the killing moment, I think,is the disarming charm. So no one tried to turn Voldy into anything nor did Neville triy to stab him with the Gryffindor sword.