I prefer "thought-stopper" better. I would say Totschlagargument sounds like it could just be a really compelling argument that leaves the other side speechless

I prefer "thought-stopper" better. I would say Totschlagargument sounds like it could just be a really compelling argument that leaves the other side speechless and "kills" the discussion with a victory; but the point is not that the argument is compelling but rather it tries to block substantive thought and discussion.