Do You Tell a Job Candidate How Badly They Did?

skelter asks: "I have been lamenting with friends in the industry about interviewing woes and the candidates that we find. Consider a hypothetical job candidate comes in after some how making it through screening. In the team technical interview they prove beyond a shadow of a doubt that not only is he (or she) not as adequate as he thinks he is, but has demonstrated that he is a danger to any code base. Do you tell them? Quietly step away, usher them out and say nothing? Play with them on the whiteboard the way your cat plays with injured mice? Should you leave them as their own warning to others? Is there any obligation to guide them to gaining real experience? Can you give them any advice or is it all liability?"