I don't think Boromir was persuadable. I don't know whether he could have left Frodo of his own choice with the hold the ring... or the idea of the ring had on him which is why he acted when it was clear perhaps he would have to let it go. And I don't think Galadriel would have held him against his will. You can't punish people for something they might do - PJ O Rourke said something on the lines of the Supreme Court can't punish you for having a smart look on your face - which is the difference between having a judicial system and having a mother....
