I have found a conversation/discussion "stopper" to be something very simple:
Ask question: Are Christians allowed to eat other people for nutrition?
Answer: Of course not. That is cannabalism.
Next question: Then is cannabalism a sin?
Answer: Of course.
Next question: Then why can Christians have organ transplants? The organ is put into your body which is OK but eating it is a sin.
Closing argument: So why do you consider having blood put into your body the same as eating it?
Game point. (The justifications can be very interesting indeed).
Rub a Dub