The correct response, the ONLY correct response is fascists get nothing. Not respect, not debate, not companionship, not an argument, not community, not safety, not kindness, nothing. It would be best if you gave them a taste of your boot, but I understand that's not always possible every time. What you cannot do, what you mustn't do, is create the impression that their arguments are up for discussion. Nothing is stopping you from shouting them down, calling them nazis, booing them out the room.