Hi there, for me the best way to end a bad interview is to thank them for their time and interest. Let them know that at this moment their qualifications doesn’t meet the requirement but you can also let them know that you’ll keep their records if in case they’ll be qualified in any available position in the future. Usually the interviewee already knows that they won’t be accepted specially when they’re doing bad during the interview but by thanking them for their time and letting them know you’re keeping their application for future reference will lessen their heartache.